Delayed email - LED=450 4.4.316 Connection refused

Question

We are hybrid with adsync. Exchange 2019 in a DAG behind load balancer. Our users are in exchange online. we have a few mailboxes on premise. When our online users send an email to the on-premises mailbox, there is a delay with the email being delivered to the mailbox (2 hours sometimes). This started about 2 weeks ago. I did a message trace. The following is the message from the trace.

4/1/2024, 4:06 PM | Defer | Reason: [{LED=450 4.4.316 Connection refused [Message=Socket error code 10061] [LastAttemptedServerName=.abc.domain [LastAttemptedIP=xxx.xxx.xxx.xxx:25] [SmtpSecurity=-2;-2] [xxxxxx04FT004.eop-NAM04.prod.protection.outlook.com 2024-04-01T20:58:43.377Z ]};{MSG=Socket error code 10061};{FQDN=abc. OutboundProxyTargetIP: xxx.xxx.xxx.xxx:. OutboundProxyTargetHostName: abc.domain

4/1/2024, 5:44 PM | Send external | Message sent to abc.domain at xxx.xxx.xxx.xxx using TLS1.2 with AES256

What would cause the delay? Thank You.

Answer 1

Hi @mara2021,

Based on your description of the problem and the message trace offered, A likely cause for this error is your firewall is blocking connections from Microsoft 365 IP addresses. Afterwards, here's a solution you can try to fix the problem: Modify your firewall settings to allow connections from Microsoft 365 IP addresses on TCP port 25 to your on-premises email servers instead of allowing only specific IPs. You can refer to the documentation linked below for a list of the Microsoft 365 IP addresses: Microsoft 365 URLs and IP address ranges - Microsoft 365 Enterprise | Microsoft Learn.

In addition, you also can view delayed mail’s message headers to analyze delay reason:

-View message headers in Outlook on your desktop

1. Double-click an email message to open it outside of the Reading Pane.
2. Click File > Properties.
3. Header information appears in the Internet headers box.

-Copy the headers information into Message Header Analyzer (Message Header Analyzer (mha.azurewebsites.net)) and check the hop’s delay details.

Hope these will be helpful for you!