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How to Create an Alias for a Plus-Addressed Exchange Online Mailbox
Question details
The user needs to configure a mail-flow design that securely routes emails to a plus-addressed GitLab mailbox without triggering mail loops in Exchange Online.
- Product
- Microsoft Exchange Online
- Device & OS
- not provided
- Scenario
- Setting up email routing aliases for ticketing systems using plus-addressed emails via an Exchange Online contact or distribution list.
- Observed behavior
- Routing emails through an Exchange Online contact or distribution list causes a mail loop because the system incorrectly processes the plus address as an external recipient.
Ensure you have administrator privileges for the Exchange Admin Center (EAC) to create shared mailboxes and modify mail-flow rules for your organization.
Set Up a Shared Mailbox and Mail-Flow Redirect Rule
Use an internal shared mailbox paired with a server-side redirect rule to ensure proper delivery without triggering an external recipient mail loop.
This configuration uses Exchange Online's built-in mail-flow rules to redirect incoming messages smoothly, preventing the directory from treating the ticketing destination as a looping external recipient.
Log in to the Exchange Admin Center, navigate to Recipients > Shared mailboxes, and create a new shared mailbox (or mail-enabled recipient) with a standard, short email address.
From the left-hand navigation menu in the Exchange Admin Center, select Mail flow and then click on Rules.
Create a new rule. Set the condition to trigger when a message is sent to the newly created shared mailbox. Set the action to 'Redirect the message to...' and specify your external plus-addressed GitLab destination.
Save the mail-flow rule, send a test email to the shared mailbox, and confirm that your ticketing system receives and processes the message without generating loop errors.
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Frequently Asked Questions
Why does my plus-addressed email cause a mail loop in Exchange Online?
When a plus-addressed email is added as an external contact within a distribution list, Exchange Online may repeatedly attempt to route the message externally without resolving the final destination, resulting in an infinite delivery loop.
Can I use a standard inbox rule instead of a global Exchange mail-flow rule?
Yes, if you assign a license to a standard user mailbox, you can configure an inbox rule to forward messages. However, using an unlicensed shared mailbox combined with a global Exchange mail-flow rule is the industry standard for ticketing systems to avoid unnecessary licensing costs.
Does Exchange Online natively support plus addressing?
Yes, Exchange Online supports plus addressing (subaddressing) by default. However, utilizing them within complex distribution list configurations or nested external contacts can still confuse the routing agent, which is why a direct mail-flow redirect rule is the recommended solution.
