Microsoft 365 shared mailbox: licence, size and Send As

A Microsoft 365 shared mailbox is a support address, such as support@yourstore.com, that several people open from their own Outlook and reply from. The mailbox itself needs no licence up to 50 GB, but everyone who uses it needs their own licensed mailbox, and they need the Send As permission to reply as the store rather than as themselves.

Below: what it costs, how to create one, which permission to give, the limits Microsoft sets, and what goes wrong when a printer, an app or a helpdesk tool tries to send from it. Every rule is taken from Microsoft's own documentation.

What it costs and how big it can get

Microsoft's licensing rules for shared mailboxes come down to four points:

• The shared mailbox is free up to 50 GB. In Microsoft's words: "A shared mailbox can store up to 50 GB of data without assigning a license to it."
• Every person who uses it needs a licence. "To access a shared mailbox, a user must have a licensed Exchange Online mailbox, but the shared mailbox doesn't require a separate license."
• Your plan must include email. Microsoft notes that Microsoft 365 Apps for business doesn't include email, while Microsoft 365 Business Standard does.
• More than 50 GB needs a licence on the mailbox. An Exchange Online Plan 2 licence raises the limit to 100 GB. Archiving and litigation hold also need a licence on the shared mailbox.

Watch the size. Microsoft warns that when a shared mailbox reaches its storage limit, it can receive email for some time but can't send. "After some time, the shared mailbox stops receiving email and senders to the mailbox receive a non-delivery receipt." For a support address, that means customers' emails bounce.

How to create a shared mailbox

You need an account with the Exchange admin role.

1. In the Microsoft 365 admin center, go to Teams & Groups → Shared mailboxes. Select "Show all" in the left pane if you don't see Teams & Groups.
2. Select "+ Add a shared mailbox", enter a name, and check the email address it suggests.
3. Select "Save changes". Microsoft says it may take a few minutes before you can add members.
4. Under "Next steps", select "Add members to this mailbox" and add the people who answer support.

Microsoft says every new shared mailbox has sign-in blocked by default, and to keep it that way: "Always block sign-in for the shared mailbox account and keep it blocked." The account has a system-generated password that nobody is meant to know. People open the shared mailbox from their own account; with automapping, which is on by default, it appears in their Outlook after they close and restart it.

If you try to send from a brand-new mailbox and get "You do not have the permission to send the message on behalf of the specified user", Microsoft says this is replication delay: wait an hour and try again.

Full Access, Send As or Send on Behalf

These three permissions decide what each person can do. They're separate: reading the mailbox doesn't let someone send from it.

Permission	What it allows	What the customer sees
Full Access	Open the mailbox and read, change and delete its email. Not sending.	Nothing; it doesn't send
Send As	Send email as the shared mailbox. Doesn't allow reading it.	The shared mailbox only. "There's no indication that the message was sent by the delegate."
Send on Behalf	Send email from the mailbox, showing who sent it	"Person on behalf of mailbox" in the From line; replies still go to the mailbox

For store support, give each person Full Access and Send As. Microsoft's own setup note says "Both permissions are required for successful shared mailbox operation." If someone has both Send As and Send on Behalf, Send As is always used.

Two details that trip people up:

• In the Exchange admin center, Send on Behalf isn't offered for shared mailboxes. Microsoft's example sets it with PowerShell instead, using Set-Mailbox with -GrantSendOnBehalfTo.
• Send As and Send on Behalf don't work in the Outlook desktop app if the shared mailbox is hidden from the address list.

To change permissions later: Exchange admin center → Recipients → Mailboxes → select the shared mailbox → Mailbox permissions → Manage mailbox delegation.

The limits Microsoft sets

Limit	Microsoft's rule
People per shared mailbox	"A shared mailbox supports a maximum of 25 users." More than that, use a Microsoft 365 group.
People outside your business	Not allowed. You can't give someone with, say, a Gmail account access.
Deleting email	You can't stop members deleting messages. To restrict deletion, Microsoft suggests a Microsoft 365 group.
Sending volume	Sending from another mailbox through permissions counts against the person sending, not the shared mailbox. The recipient rate limit is 10,000 recipients per day.
Encryption	Email sent from a shared mailbox can't be encrypted, because the mailbox has no security context of its own.

When a printer, app or helpdesk sends as the shared mailbox

Outlook opens a shared mailbox through the member's own account. Anything that sends email by logging in to Microsoft's SMTP server, smtp.office365.com, is different: it signs in with a user name and password, and a shared mailbox's own account isn't meant to sign in.

Microsoft's documentation describes what to do. The device or app signs in as a licensed mailbox, and if it sends from a different address, "the sign-in account needs Send As permission in the mailbox". Without it, the email bounces with:

5.7.60 SMTP; Client doesn't have permissions to send as this sender.

Two more things to check before you set this up:

• SMTP AUTH must be allowed for the mailbox that signs in. Microsoft notes it "is disabled for organizations created after January 2020 but you can enable it per-mailbox", and that it is already off if your organisation uses Security defaults.
• Basic authentication for SMTP AUTH is being switched off. Microsoft disables it by default for existing tenants at the end of December 2026. See Microsoft's SMTP AUTH deprecation, explained for the dates and the options.

Forwarding the shared mailbox to another tool

If you forward support@ to a helpdesk or shared inbox tool outside Microsoft 365, two settings matter:

1. Mailbox forwarding. Admin center → Users → Active users → the mailbox → Mail → Manage email forwarding. Microsoft says forwarding needs a licence on the "from" account "unless it's a shared mailbox".
2. Your outbound spam policy. Automatic forwarding to outside addresses is controlled there. Microsoft says the default setting, "Automatic - System-controlled", changed in 2021 to mean "Off - Forwarding is disabled" for new organisations and for existing ones that weren't using it, and that disabled forwarding "results in a non-delivery report". Ask your admin to check the policy before you rely on the forward; Microsoft recommends choosing "On - Forwarding is enabled" or "Off - Forwarding is disabled" explicitly rather than the automatic value.

A note to send your Microsoft 365 admin

If someone else manages your Microsoft 365, this asks for the right setup in one go:

Hi {{admin_name}},

Could you set up a shared mailbox for customer support, please?

Address: {{support_address}}

People who answer support (each needs Full Access and Send As): {{team_member_addresses}}

{{forwarding_line}}

Thanks,

{{your_name}}

For {{forwarding_line}}, write either "No forwarding needed" or "Please forward it to {{forwarding_address}} and allow automatic external forwarding for this mailbox."

Where Corresso fits

If you forward your Microsoft 365 support address to Corresso, the mailbox stays where it is: your mail stays with your provider, and replies go out from your own address through your provider's mail server.

Corresso is a shared support inbox for small online stores. Every incoming email gets a draft reply written from documents your store approved, such as your returns policy and shipping page, and each fact in a draft cites the document it came from. Rules written in code hold a draft that promises a refund without citing a document allowed to offer one. A person always reads the draft and presses Send; there is no auto-send. How it works, how it works with Shopify, and pricing.

On Google Workspace instead? There's no shared mailbox type there; see shared mailbox options in Google Workspace.