Running a 2003 Exchange Server with a 900+ member distribution list made up of contacts. The list only accepts emails from list members. If I send an email from inside my domain to the list, the email processes without issue.
If a member from the outside sends an email message to the list, one copy always stays in the Message Queued for Deferred Delivery. The message then resends to most list members. I have confirmed that members are receiving the message several times.
This worked for the past 5 years and only within the past 3 weeks has had issues. No changes have been made to the Exchange Server.
Also using MailEssentials to filet SPAM.