How to forward email to Plain from a Microsoft 365 distribution group
If your support address, such as support@yourcompany.com, is a Microsoft Exchange distribution group, set forwarding directly on the group instead of creating a transport rule. Transport rules cannot use the SentTo condition to match distribution groups, so you will see this error if you try:
System.ArgumentException: SentTo predicate does not allow distribution groups
This is an Exchange limitation, not a Plain error.
Option 1: Set forwarding on the distribution group
- Sign in to the Exchange Admin Center at admin.exchange.microsoft.com.
- Go to Recipients, then Groups.
- Select your distribution group, such as
support@yourcompany.com. - Open the Settings or Delivery management tab.
- Enable message forwarding and enter your Plain inbound address. You can find it in Settings, under Email, in your Plain workspace. It ends with
@inbound.postmarkapp.com. - Save the group.
Option 2: Convert the group to a shared mailbox
If you need a transport rule for extra routing logic, convert the distribution group to a shared mailbox first. Shared mailboxes support the SentTo condition, so you can then create a standard transport rule:
- Condition: The recipient is your support address.
- Action: Redirect the message to your Plain inbound address.
Converting a distribution group to a shared mailbox changes how membership and permissions work in Exchange. Check with your IT admin before making this change.