Forward a Microsoft 365 shared mailbox to Slack

Steps

1. 1

   Allow external forwarding for the mailbox

   In the Microsoft Defender portal (security.microsoft.com), go to Email & collaboration > Policies & rules > Threat policies > Anti-spam and select Create policy > Outbound. Add the shared mailbox under Users, and on Protection settings set Automatic forwarding rules to "On - Forwarding is enabled". Create the policy.

2. 2

   Open the shared mailbox

   In the Microsoft 365 admin center, go to Teams & groups > Shared mailboxes, select the mailbox, and select Edit under Email forwarding.

3. 3

   Forward to RelayMail

   Check "Forward all emails sent to this mailbox", enter the forwarding address from your RelayMail setup page (inbox-...@relay.relaymail.app), and Save. In the Exchange admin center, the same setting is under Recipients > Mailboxes > the mailbox > Manage email forwarding; there you can also keep a copy in the mailbox.

4. 4

   Check remote domains

   If forwarding still bounces, check Remote domains in Exchange: automatic forwarding to that domain must be allowed. A block there wins over an outbound policy that allows it.

5. 5

   Send a test email

   Email the shared address from another account. When it arrives, RelayMail confirms forwarding by itself and the email appears in the channel as a thread; reply there and the sender gets your reply by email.

Confirming the forward

Microsoft sends no confirmation email for forwarding; it applies to new messages once saved, and RelayMail confirms forwarding by itself when the first test email arrives. If the test never arrives, the forward is probably blocked (see below). Replies you write in the Slack thread go out from noreply@relay.relaymail.app with Reply-To set to your support address, so the customer's answer comes back through the same forward into the same thread. To send replies as support@ itself, verify your domain in RelayMail (see the custom domain guide); that adds sending records for RelayMail and does not change where your inbound mail goes.

What to watch for

• Automatic external forwarding is off by default for most tenants. Blocked forwards bounce to the sender with "5.7.520 Access denied, Your organization does not allow external forwarding".
• When one control allows external forwarding and another blocks it (outbound spam policy, remote domains, mail flow rules), the block usually wins.
• Exchange Online rewrites the envelope sender (SRS) and implements ARC; the From line is unchanged. Mail from domains with p=reject DMARC can still be rejected downstream if the original DKIM signature does not survive.
• Shared mailboxes need no license to forward. The admin center field takes one address; use a distribution group if you need more than one destination.
• Forwarding passes spam and phishing along too; Microsoft notes the destination needs its own filtering.

Questions