We have an SBS 2011 with Exchange 2010 and all of a sudden we we're not receiving any inbound email from the internet. Senders are reporting the following NDR:
Unknown recipient channel
A problem occurred while delivering this message to this email address. Try sending this message again. If the problem continues, please contact your helpdesk.
Event viewer on the server shows the following errors:
Receive connector 127.0.0.1:25 requires Transport Layer Security (TLS) before the MailFrom command can be run, but the server can't achieve it. Check this connector's authentication setting.
The only thing was changed was anti-spam content filtering.
Can anyone help to get this resolved?