Why does my distribution group not accept emails from external senders?

By default, Exchange distribution groups require senders to be authenticated (i.e. internal Exchange users). External senders — including emails forwarded by POPcon — are rejected.

Fix

Option A: Exchange Management Console

1. Open the distribution group properties.
2. Go to Mail Flow Settings > Message Delivery Restrictions.
3. Uncheck "Require that all senders are authenticated".

Option B: Exchange Management Shell

Set-DistributionGroup GROUPNAME -RequireSenderAuthenticationEnabled $False

Screenshots — step by step