Messages sent to a large number of recipients get stuck in the delivery queue and are not delivered to even a single recipient. Flushing the delivery queue has no effect on these messages.
Messages that get stuck in the delivery queue show the following error:
451 4.4.2 [internal] connection closed by remote host
Two headers (x-cr-puzzleid and x-cr-hashedpuzzle) inserted by Microsoft Outlook 2007 have been identified to cause delivery failures of certain messages sent to multiple recipients. These two headers are due to the “Email Postmarking” feature in Outlook 2007. Some messages sent from Outlook 2007 with the Postmarking option on (by default) to multiple recipients contain the above mentioned two headers and at least one of those headers may exceed the maximum line length of 998 characters allowed by RFC standards. SMG faithfully tries to deliver these messages with long header lines to the recipients without making any modifications to the original header lines. Some firewalls, with mail security features enabled, do not like such headers and drop the connection without returning any error message.
Since the issue is environmental with mail agents other than SMG involved in the delivery chain, following workarounds may help resolve the issue:
Please turn off the Postmarking feature of Outlook 2007 off by unchecking the “When sending e-mail, postmark the message to help recipient e-mail programs distinguish regular e-mail from junk e-mail” option in Outlook 2007 (under Tools > Options > Preferences > E-mail > Junk E-mail).
Please turn off the SMTP Protocol Inspection feature in the firewall.
Microsoft Exchange Server
Firewall with mail inspection features like Cisco PIX