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What to create: Office 365 groups, shared mailboxes, security groups, and distribution lists

In the Groups section of the Office 365 admin center, you can create and manage these types of groups:

Option
When to choose
Office 365 group
Create an Office 365 group when you want to give people a place to collaborate. When you create a group, you automatically give its members permissions to a shared mailbox, calendar, files, SharePoint site, and more. Learn about Office 365 Groups.
You can also create a group if you want an email address like info@contoso.com.
Advantages of groups:
They are easy to create! Users create them in Outlook. Admins create them in the admin center.
You can grant guests outside of your company access to the groups. For example, a partner with a Gmail address. To learn more, see Guest access in Office 365 Groups
Users can use their mobile device to access/manage email sent to the group.
People outside your organization can email the group. So you can have an address like info@contoso.com.
Messages sent to the group are preserved for everyone, even if one person "deletes" the message after reading it in their Outlook inbox. Only a Group owner can delete a message from the Group inbox. See How following group email works.
User's can subscribe and unsubscribe to group email, notifications, etc. See How following group email works.
Two drawbacks, at least compared to shared mailboxes: If you decide to use a group for something like info@contoso.com ...
You can allow group members to reply as the group address instead of their personal email address. BUT, it doesn't do it by default. You have to remind your users to always choose Send As the group mailbox.
After you create the group, you have to enable it to Let people outside the organization email the group to your business.
Shared mailboxes
Create a shared mailbox when you want a group of people to monitor and send email from a common email address, like info@contoso.com.
When a person in the group replies to a message sent to the shared mailbox, the email appears to be from the shared mailbox, not from the individual user.
Notable benefits of shared mailboxes:
You don't need to assign licenses to mailboxes that are under 50 GB.
Users can reply as the shared mailbox address instead of their personal email address.
Shared mailboxes include a shared calendar. If you're a small business and you need to keep track of what appointments people are on, this is a good way for everyone to be informed.
You can convert user mailboxes to shared mailboxes.
A few drawbacks:
You can't access a shared mailbox from Outlook for iPhone, Android, or Mac. If you want to access a shared mailbox from one of these devices, open a browser and go to Outlook on the web.
Unlike Office 365 Groups, only members inside your organization can read or reply to email sent to a shared mailbox. You can't add a partner who has a Gmail address, for example, as a member of a shared mailbox.
Say you have 5 people sharing a mailbox. One of them deletes a message after reading it. Now it's been deleted for the other 4 people too, even though they hadn't read the message. To avoid this scenario, use Groups instead; read the section on How following group email works.
Distribution list
It's better to create an Office 365 group than a distribution list. The only reason to create a DL now is if your organization has Groups disabled.
You can upgrade existing distribution lists to Office 365 Groups in Outlook.
Security group
Create a security group when you want to grant access permissions to a group of users. For example, you could create a group of designers for a SharePoint Online site and then assign read/write permissions to that group. Everyone in the group will be able to view and edit the site.
Another example is when setting up mobile device management (MDM), a security group is created for the security policy. Assigning permissions to groups is much faster and more efficient than assigning permissions to individual users one at a time.
If you have hundreds or thousands of users you're managing, you might be interested in using a security group to assign licenses. See Assign licenses to users by group membership in Azure Active Directory.
Mail-enabled security group
If you mail-enable the security group, you can send e-mail to all members of that group. For example, if you create a security group that gives members access to the computers in a specific server lab, you may want to send mail to that group to notify them of a power outage in the lab.
Note: You have to assign Exchange Online licenses to all users who you want to add to a mail-enabled security group.
To learn more, see Manage mail-enabled security groups.

IMPORTANT: Currently it's not possible to migrate a shared mailbox to an Office 365 Group. Is this something you want? Let us know. Vote here !

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