If you have an ISA 2006 server between a Microsoft Exchange 2007 Edge server and the Exchange Hub Transport server, you may have a problem where messages queue on the Edge with 500 5.1.1 "unrecognized command" errors.
This Microsoft article partially explains how to resolve the problem. When the Edge Transport server tries to send mail through Microsoft Internet Security and Acceleration (ISA) Server 2006, with SMTP filtering or Secure SMTP (SMTPS) filtering enabled, the SMTP filter blocks the communication. You fix this by either disabling the SMTP filter on the ISA server or adding the verbs (and optionally their maximum length) to the SMTP filter.
What the article doesn't say is which verbs to add or their maximum length. Well, here they are:
- X-EXPS NTLM
All the verbs have an empty maximum length except for possibly SIZE. That should be set to the maximum message size allowed in your org in bytes (for example, 10485760 for 10MB).