Message size limits in Exchange Server

You can apply limits to messages that move through your organization. You can set the maximum size of an entire message as a whole, or the size of individual parts of a message, or both. For example, you could restrict the maximum size of the message header or attachments, or set a maximum number of recipients that can be added to the message. You can apply these limits to your entire Exchange organization, to specific mail transport connectors, specific servers, and to individual mailboxes.

This topic only talks about message and recipient size limits. If you want to know more about how to control how many messages are sent over time, how many connections are allowed over time, and how long Exchange will wait before closing a connection, see Message rate limits and throttling.

As you plan the message size limits for your Exchange organization, consider the following questions:

  • What size limits should I impose on all incoming messages?

  • What size limits should I impose on all outgoing messages?

  • What is the mailbox quota for my organization, and how do the message size limits that I have chosen relate to the mailbox quota size?

  • Are there users in my organization who need to send or receive messages that are larger than the maximum allowed size?

  • Does my organization include other messaging systems or separate business units that require different message size limits?

This topic provides guidance to help you answer these questions and to apply the appropriate message size limits in the appropriate locations.

Types of message size limits

The following list describes the basic types of message size limits, and the message components that they apply to.

  • Whole message size limits: Specifies the maximum size of a message, which includes the message header, the message body, and any attachments. Exchange uses the custom X-MS-Exchange-Organization-OriginalSize: message header to record the original size of the message as it enters the Exchange organization. Whenever the message size is checked, the lower value of the current message size or the original message size header is used. The size of the message can change because of content conversion, encoding, and transport agent processing.

    For any message size limit, you need to set a value that's larger than the actual size you want enforced. This accounts for the Base64 encoding of attachments and other binary data. Base64 encoding increases the size of the message by approximately 33%, so the value you specify should be approximately 33% larger than the actual message size you want enforced. For example, if you specify a maximum message size value of 64 MB, you can expect a realistic maximum message size of approximately 48 MB.

  • Attachment size limits: Specifies the maximum size of a single attachment in a message. The message might contain many smaller attachments that greatly increase its overall size. However, the attachment size limit applies only to the size of an individual attachment. While you can't limit the number of attachments on a message, you can use the maximum message size limit to control the maximum total of attachments on the message.

  • Recipient limits: Specifies the total number of recipients that are allowed in a message. This includes the total number of recipients in the To:, Cc:, and Bcc: fields. A distribution group counts as a single recipient.

  • Message header size limits: Specifies the maximum size of all message header fields in a message. The size of the message body or attachments isn't considered. Because the header fields are plain text, the size of the header is determined by the number of characters in each header field and by the total number of header fields. Each text character consumes 1 byte.

Scope of limits

The following tables show the message limits at the Organization, Connector, Server, and Mailbox levels, including information about how to configure the limits in the Exchange admin center (EAC) or the Exchange Management Shell. To learn how to open the Exchange Management Shell in your on-premises Exchange organization, see Open the Exchange Management Shell.

Organizational limits

Organizational limits apply to all Exchange 2019 servers, Exchange 2016 servers, Exchange 2013 Mailbox servers, and Exchange 2010 Hub Transport servers that exist in your organization. On Edge Transport servers, any organizational limits that you configure are applied to the local server.

Size limit Default value EAC configuration Exchange Management Shell configuration
Maximum size of a message received 10 MB Mail flow > Receive connectors > More options More Options icon > Organization transport settings > Limits tab > Maximum receive message size (MB) Cmdlet: Set-TransportConfig
Parameter: MaxReceiveSize
Maximum size of a message sent 10 MB Mail flow > Receive connectors > More options More Options icon > Organization transport settings > Limits > Maximum send message size (MB) Cmdlet: Set-TransportConfig
Parameter: MaxSendSize
Maximum number of recipients in a message 5000 Mail flow > Receive connectors > More options More Options icon > Organization transport settings > Limits Maximum number of recipients Cmdlet: Set-TransportConfig
Parameter: MaxRecipientEnvelopeLimit
Maximum attachment size for a message that matches the conditions of the Transport rule Not configured Mail flow > Rules > Add Add icon > Create a new rule, or select an existing rule, and then click Edit Edit icon.
Click More options.
Use the condition Apply this rule if > The message > size is greater than or equal to, and enter a value in kilobytes (KB).
Cmdlets: New-TransportRule, Set-TransportRule
Parameter: AttachmentSizeOver
Maximum message size for a message that matches the conditions of the Transport rule Not configured Mail flow > Rules > Add Add icon > Create a new rule, or select an existing rule, and then click Edit Edit icon.
Click More options.
Use the condition Apply this rule if > The message > size is greater than or equal to, and enter a value in kilobytes (KB).
Cmdlets: New-TransportRule, Set-TransportRule
Parameter: MessageSizeOver

To see the values of these organizational limits, run the following commands in the Exchange Management Shell:

Get-TransportConfig | Format-List MaxReceiveSize,MaxSendSize,MaxRecipientEnvelopeLimit
Get-TransportRule | where {($_.MessageSizeOver -ne $null) -or ($_.AttachmentSizeOver -ne $null)} | Format-Table Name,MessageSizeOver,AttachmentSizeOver

Connector limits

Connector limits apply to any messages that use the specified Send connector, Receive connector, Delivery Agent connector, or Foreign connector for message delivery.

You can assign specific message size limits to the Active Directory site links in your organization. The Transport service on Mailbox servers uses Active Directory sites, and the costs that are assigned to the Active Directory IP site links as one of the factors to determine the least-cost routing path between Exchange servers in the organization.

You can assign specific message size limits to the Delivery Agent connectors and Foreign connectors that are used to send non-SMTP messages in your organization.

Size limit Default value EAC configuration Exchange Management Shell configuration
Maximum size of a message sent through the Receive connector 36 MB Mail flow > Receive connectors > Edit Edit icon > General > Maximum receive message size (MB) Cmdlets: New-ReceiveConnector, Set-ReceiveConnector
Parameter: MaxMessageSize
Maximum size of all header fields in a message sent through the Receive connector 256 KB Not available Cmdlets: New-ReceiveConnector, Set-ReceiveConnector
Parameter: MaxHeaderSize
Maximum number of recipients in a message sent through the Receive connector Transport service on Mailbox servers
Default <ServerName>: 5000
Client Proxy <ServerName>: 200
Front End Transport service on Mailbox servers
Default Frontend <ServerName>: 200
Outbound Proxy Frontend <ServerName>: 200
Client Frontend <ServerName>: 200
If the number of recipients is exceeded in a message from an anonymous sender (for example, an Internet sender), the message is accepted for the first 200 recipients. Most messaging servers will continue to resend the message in groups of 200 recipients until the message is delivered to all recipients.
Not available Cmdlets: New-ReceiveConnector, Set-ReceiveConnector
Parameter: MaxRecipientsPerMessage
Maximum size of a message sent through the Send connector 10 MB Mail flow > Send connectors > Edit Edit icon > General tab > Maximum send message size (MB) Cmdlets: New-SendConnector, Set-SendConnector
Parameter: MaxMessageSize
Maximum size of a message sent through the Active Directory site link Unlimited Not available Cmdlet: Set-AdSiteLink
Parameter: MaxMessageSize
Maximum size of a message sent through the Delivery Agent connector Unlimited Not available Cmdlets: New-DeliveryAgentConnector, Set-DeliveryAgentConnector
Parameter: MaxMessageSize
Maximum size of a message sent through the Foreign connector Unlimited Not available Cmdlet: Set-ForeignConnector
Parameter: MaxMessageSize

To see the values of these connector limits, run the following command in the Exchange Management Shell:

Get-ReceiveConnector | Format-Table Name,Max*Size,MaxRecipientsPerMessage; Get-SendConnector | Format-Table Name,MaxMessageSize; Get-AdSiteLink | Format-Table Name,MaxMessageSize; Get-DeliveryAgentConnector | Format-Table Name,MaxMessageSize; Get-ForeignConnector | Format-Table Name,MaxMessageSize

Server limits

Server limits apply to specific Mailbox servers or Edge Transport servers. You can set these message size limits independently on each Mailbox server or Edge Transport server.

Size limit Default value EAC configuration Exchange Management Shell configuration
Maximum size for a message sent by Outlook on the web clients 35 MB Not available You configure this value in web.config XML application configuration files on the Mailbox server. For more information, see Configure client-specific message size limits.
Maximum size for a message sent by Exchange ActiveSync clients 10 MB Not available You configure this value in web.config XML application configuration files on the Mailbox server. For more information, see Configure client-specific message size limits.
Maximum size for a message sent by Exchange Web Services clients 64 MB Not available You configure this value in web.config XML application configuration files on the Mailbox server. For more information, see Configure client-specific message size limits.

The pickup directory that's available on Edge Transport servers and Mailbox servers also has messages size limits that you can configure. Typically, the pickup directory isn't used in everyday mail flow. It's is used by administrators for mail flow testing, or by applications that need to create and submit their own messages files. For more information, see Configure the Pickup Directory and the Replay Directory.

  • Maximum size of all header fields in a message file placed in the pickup directory: 64 KB.

  • Maximum number of recipients in a message file placed in the pickup directory: 100.

Recipient limits

Recipient limits apply to a specific user object, such as a mailbox, mail contact, mail user, distribution group, or a mail-enabled public folder.

Size Limit Default value EAC configuration Exchange Management Shell configuration
Maximum size of a message that can be sent to the specific recipient Site mailbox provisioning policies: 36 MB
All other recipient types: unlimited
For mailboxes:
Recipients > Mailboxes > Edit Edit icon > Mailbox features > Mail flow section > Message size restrictions section > View details > Received messages section > Maximum message size (KB)
For mail users:
Recipients > Contacts > Edit Edit icon > Mail flow settings > Message size restrictions > View details > Received messages section > Maximum message size (KB)
This setting available in the EAC for other types of recipients.
Cmdlets:
Set-DistributionGroup
Set-DynamicDistributionGroup
Set-Mailbox
Set-MailContact
Set-MailUser
Set-MailPublicFolder
New-SiteMailboxProvisioningPolicy
Set-SiteMailboxProvisioningPolicy
Parameter: MaxReceiveSize
Maximum size of a message that can be sent by the specific sender Unlimited For mailboxes:
Recipients > Mailboxes > Edit Edit icon > Mailbox features > Mail flow section > Message size restrictions section > View details > Sent messages section > Maximum message size (KB)
For mail users:
Recipients > Contacts > Edit Edit icon > Mail flow settings > Message size restrictions section > View details > Sent messages section > Maximum message size (KB)
This setting available in the EAC for other types of senders.
Cmdlets:
Set-DistributionGroup
Set-DynamicDistributionGroup
Set-Mailbox
Set-MailContact
Set-MailUser
Set-MailPublicFolder
Parameter: MaxSendSize
Maximum number of recipients in a message that's sent by the specific sender Unlimited For mailboxes:
Recipients > Mailboxes > Edit Edit icon > Mailbox features > Mail flow section > View details > Recipient limit section > Maximum recipients
This setting isn't available in the EAC for mail users.
Cmdlets:
Set-Mailbox, Set-MailUser
Parameter: RecipientLimits

To see the values of these limits, run the corresponding Get- cmdlet for the recipient type in the Exchange Management Shell.

For example, to see the limits that are configured on a specific mailbox, run the following command:

Get-Mailbox <MailboxIdentity> | Format-List MaxReceiveSize,MaxSendSize,RecipientLimits

To see the limits that are configured on all user mailboxes, run the following command:

$mb= Get-Mailbox -ResultSize unlimited; $mb | where {$_.RecipientTypeDetails -eq 'UserMailbox'} | Format-Table Name,MaxReceiveSize,MaxSendSize,RecipientLimits

Order of precedence and placement of message size limits

The order of precedence for message size limits is the most restrictive limit is enforced. The only question is where that limit is enforced. The goal is to reject messages that are too large as early in the transport pipeline as possible. For example, it's a waste of system resources for the Internet Receive connector to accept large messages that are eventually rejected because of a lower organizational limit. Make sure that your organization, server, and connector limits are configured in a way that minimizes any unnecessary processing of messages. You do this by keeping the limits the same in all locations, or by configuring more restrictive limits where messages enter your Exchange organization.

An exception to the order is message size limits on mailboxes and messages size limits in mail flow rules (also known as transport rules). Exchange checks the maximum message size that's allowed on mailboxes before mail flow rules process messages. For example, your organization's message size limit is 50 MB, you configure a 35 MB limit on a mailbox, and you configure a mail flow rule to find and reject messages larger than 40 MB. If an external sender sends a 45 MB message to the mailbox, the message is rejected before the mail flow rule is able to evaluate the message.

Recipient limits between authenticated senders and recipients (typically, internal message senders and recipients) are exempt from the organizational message size restrictions. Therefore, you can configure specific senders and recipients to exceed the default message size limits for your organization. For example, you can allow specific mailboxes to send and receive larger messages than the rest of the organization by configuring custom send and receive limits for those mailboxes.

However, this exemption applies only to messages sent between authenticated senders and recipients (typically, internal senders and recipients). For messages sent between anonymous senders and recipients (typically, Internet senders or Internet recipients), the organizational limits apply. For example, suppose your organizational message size limit is 10 MB, but you configured the users in your marketing department to send and receive messages up to 50 MB. These users will be able to exchange large messages with each other, but not with Internet senders and recipients (unauthenticated senders and recipients).

Messages exempt from size limits

The following list shows the types of messages that are generated by Mailbox servers or Edge Transport servers that are exempted from all message size limits except the organizational limit for the maximum number of recipients that are allowed in a message:

  • System messages

  • Agent-generated message

  • Delivery status notification (DSN) messages (also known as non-delivery reports, NDRs, or bounce messages). However, you can use the ExternalDsnMaxMessageAttachSize and InternalDsnMaxMessageAttachSize parameters on the Set-TransportConfig cmdlet to limit the size of original messages that are included in DSN messages (hence, the effective size of the DSN message itself).

  • Journal report messages

  • Quarantined messages