how to grant send-as permission on a shared mailbox

TL;DR

In the Exchange admin center, open the shared mailbox's delegation settings and add the user under Send As. The permission can take up to an hour to take effect, so most Send As is broken tickets are just propagation. Verify with Get-RecipientPermission before escalating.

Steps

1. Open Exchange admin center > Recipients > Mailboxes, select the shared mailbox, and open Delegation. Expected: the mailbox properties page loads with its delegation tabs.
2. Under Send As, click Add members and select the user or group. Expected: the name appears in the Send As list. Save.
3. Wait up to 60 minutes for propagation. Expected: no error yet; test only after the wait or you will chase a ghost.
4. The user composes a new message and picks the shared address in the From field. Expected: the recipient sees the shared mailbox as the sender, not the user's name.
5. Verify in Exchange PowerShell: Get-RecipientPermission -Identity "[shared mailbox identity]" and look for SendAs in the output. Expected: the trustee is listed with the SendAs right.

Use this when

• A user needs to send from a team or shared address
• Replies should look like they come from the shared mailbox itself
• A new team member joins and needs the same sending rights

Not for this skill when

• The user only needs to read the mailbox (grant Full Access instead)
• Replies should show sent on behalf of (that is the separate Send on Behalf permission)
• Delegation in Gmail (different product, different screens)

Compatibility

• Exchange Online; Exchange admin center and Exchange Online PowerShell v3 module
• Outlook on the web, Outlook desktop, Outlook mobile after the mailbox is added

Variants

Send As granted but still failing

Re-check the permission actually saved, then wait the full hour. Cached Outlook profiles sometimes need a restart.

Needs Send As on mobile

Add the shared mailbox to Outlook mobile first; the From picker appears once the mailbox is present.

PowerShell one-liner to grant

Add-RecipientPermission -Identity "[shared mailbox identity]" -Trustee [the user's email] -AccessRights SendAs. Expected: the command returns the new permission entry.

Why it happens

Send As is a separate permission from Full Access. Admins grant one, assume both, and the ticket arrives when the user tries to reply as the team.

Edge cases

• Dynamic distribution groups cannot receive Send As; use a mail-enabled security group instead.
• The trustee must be a licensed mailbox user.
• External senders cannot use Send As on your shared mailbox.

Provenance

Resolved from the public thread: https://vectle.com/posts/pstIyxwLjcbs4oacv82kRAnw