Somebody asks for an info@ address, or a sales@ address, or a way to email the whole crew at once. Microsoft 365 gives you three different tools that all look like "an email address a bunch of people use," and picking the wrong one is how you end up with replies coming from the wrong person, a mailbox that quietly stops accepting mail, or a list nobody can search.

Here is what each one actually does, where the mail lives, and how to build it.

The Short Version

Shared mailbox. One real inbox that several people open. Replies go out from the shared address, everyone sees the same messages, and it has its own calendar. Use it for info@, support@, billing@, and front-desk addresses.

Distribution list. A forwarding list. Mail sent to the address gets copied into every member's own inbox, and there is no shared storage at all. Use it to broadcast to a fixed set of people, like everyone in the office or the field crew.

Microsoft 365 group. A full shared workspace with a group mailbox, a shared calendar, a SharePoint file library, and a Planner board, and it is the membership behind every team in Microsoft Teams. Use it when a group of people needs to work together and keep history.

Shared Mailboxes: One Inbox, Many Hands

A shared mailbox is a normal mailbox that nobody signs in to directly. Members open it alongside their own mailbox in Outlook, and when they reply, the message comes from the shared address, not from them. That is the whole point: customers see "Support" and keep writing to Support, no matter who is on shift.

Access comes in three permissions. Full Access lets someone open the mailbox and read, delete, and change items. Send As lets them send mail that looks like it came from the mailbox itself. Send on Behalf shows the sender as "Jordan on behalf of Reception." Full Access alone cannot send anything, so most setups pair Full Access with Send As.

Here is the part that trips everyone up: by default, a message sent from the shared mailbox lands in the sender's own Sent Items folder, not the shared one. The rest of the team never sees the reply went out. Microsoft's fix is one setting per mailbox:

Set-Mailbox -Identity support@contoso.com -MessageCopyForSentAsEnabled $true
Set-Mailbox -Identity support@contoso.com -MessageCopyForSendOnBehalfEnabled $true

With that turned on, a copy of every reply also lands in the shared mailbox's Sent Items, so the whole team can see the conversation.

A few limits to know before you build one. Microsoft documents a maximum of 25 users on a shared mailbox, and past that people start seeing connection failures and duplicate messages. You cannot stop members from deleting messages. You cannot give access to anyone outside your organization. And mail sent from a shared mailbox cannot be encrypted with the mailbox's own key, because the mailbox has no security context of its own.

Shared Mailbox Licensing and the 50 GB Wall

Shared mailboxes are free up to a point. Without a license, a shared mailbox can hold 50 GB. Past that, it needs an Exchange Online Plan 2 license, which raises the limit to 100 GB. Turning on an archive or placing the mailbox on litigation hold also requires a license: Exchange Online Plan 2, or Plan 1 with the Exchange Online Archiving add-on.

Hitting the wall is not graceful. Microsoft says that when a shared mailbox reaches its storage limit, it keeps receiving mail for a while but cannot send. After some time it stops receiving too, and senders get a non-delivery report. For an info@ address, that is a silent outage until a customer complains. One quirk: unlicensed shared mailboxes created before July 2018 were provisioned at 100 GB and keep that size.

Check your shared mailbox sizes every so often from Exchange Online PowerShell:

Get-EXOMailbox -RecipientTypeDetails SharedMailbox -ResultSize Unlimited |
  Get-EXOMailboxStatistics |
  Select-Object DisplayName, TotalItemSize

Every shared mailbox also has a user account behind it. New ones come with sign-in blocked, and Microsoft says to keep it that way.

Distribution Lists: Broadcast Only

A distribution list, which Microsoft also calls a distribution group, is the simplest of the three. Send one email to the list address and every member gets their own copy. There is no shared inbox, no shared Sent Items, and no history. Someone added to the list next month will never see what was sent before they joined, and replies come from each member's own address.

That makes it the right tool for announcements: everyone at the company, everyone in one building, the on-call rotation. It can accept mail from outside your organization if you allow it. If you want membership that maintains itself, a dynamic distribution group builds the recipient list from attributes like department or office every time a message is sent.

New-DistributionGroup -Name "Field Crew" -PrimarySmtpAddress fieldcrew@contoso.com -Members "alex@contoso.com","sam@contoso.com"

# Allow outside senders, such as vendors, to email the list
Set-DistributionGroup -Identity "Field Crew" -RequireSenderAuthenticationEnabled $false

Microsoft 365 Groups: The Whole Workspace

A Microsoft 365 group gives its members a group email address plus a shared workspace: conversations, a calendar, a SharePoint file library, and a Planner board. Every team in Microsoft Teams sits on top of one. Groups can include guests from outside the company if your admin allows it, and membership can be dynamic, driven by attributes in Microsoft Entra ID.

Mail sent to a group is stored in the group's own mailbox, so a new member can scroll back through the whole conversation history. Members who subscribe get copies in their inbox too, and deleting your copy does not delete the group's. Groups are public by default, meaning anyone in the company can join. A private group requires owner approval to join, although anyone can still send email to it.

New-UnifiedGroup -DisplayName "Project Falcon" -Alias "falcon" -AccessType Private

Where Mail-Enabled Security Groups Fit

There is a fourth option that confuses things. A mail-enabled security group works like a regular security group, granting access to things like SharePoint sites, and it can also receive email that goes to every member. Use it when the same list of people should both get the email and get access to a resource. It cannot have dynamic membership and cannot contain devices, and it can be added to a team in Teams.

Creating Each One in the Admin Center

Shared mailbox: in the Microsoft 365 admin center, go to Teams & groups, then Shared mailboxes, and select Add a shared mailbox. Give it a name, save, and then use Add members to this mailbox under Next steps. Members added this way show up in Outlook automatically after a restart, through a feature called automapping.

The PowerShell version creates the mailbox and grants Full Access and Send As to one user:

New-Mailbox -Shared -Name "Support" -DisplayName "Support" -Alias support
Add-MailboxPermission -Identity support -User alex@contoso.com -AccessRights FullAccess -InheritanceType All
Add-RecipientPermission -Identity support -Trustee alex@contoso.com -AccessRights SendAs -Confirm:$false

Distribution lists and Microsoft 365 groups: go to Teams & groups, then Active teams & groups, and add the type you want. Distribution lists can also be built in the Exchange admin center under Recipients, then Groups.

Converting a Distribution List to a Microsoft 365 Group

If a distribution list has grown into a team that needs shared files and history, you can upgrade it in place. An admin starts the process in the Exchange admin center: go to Recipients, then Groups, then Distribution list, select the list, and choose Send upgrade request. The list owners get an email with an Upgrade button, and the conversion finishes within about ten minutes. The email address stays the same, so nobody has to update contacts.

Only cloud-managed, simple, non-nested lists qualify, and owners cannot start the upgrade themselves. To see which of your lists are eligible:

Get-EligibleDistributionGroupForMigration

There is no equivalent path for shared mailboxes. Microsoft states plainly that a shared mailbox cannot be migrated to a Microsoft 365 group, so if you think a mailbox will turn into a team someday, start with the group.

Common Mistakes

Signing in to the shared mailbox directly. Someone resets the shared account's password so a new hire can "just log in." That creates an unlicensed account with a known password and no MFA tied to a person. Give the person Full Access instead.

Using a shared mailbox for a big team. Past 25 users, Microsoft says to use a Microsoft 365 group. Shared mailboxes are built for a small desk, not a department.

Granting access through a security group and expecting automapping. Automapping is set on each user's mailbox, so group-based permissions will not make the shared mailbox appear in Outlook on its own. Grant access to people directly.

Letting a shared mailbox drift past 50 GB. Old attachments pile up for years. Check sizes or license the mailbox before it stops sending.

References

Want your Microsoft 365 mailboxes and groups set up cleanly the first time? Contact Rain City Techworks.