Many proxies and firewalls are sensitive to SMTP and POP3 conversations because of the potential for a buffer overflow or denial of service (DoS) attack.
In fact, at least one attack using a proprietary Exchange Server verb has already been enumerated.
That said, it doesn't make sense to block all the custom Exchange Server verbs that are used in this context, since that can break needed functionality.
Exchange Server uses three proprietary verbs: X-EXPS, X-LINK2STATE and XEXCH50. None of these Exchange Server verbs are as well-documented as they could be, and they are often blocked by firewalls or proxies that aggressively manage SMTP/POP3 traffic.
When this happens, a number of symptoms can manifest:
- Exchange servers can't authenticate each other.
- Seemingly commonplace commands are responded to with the 500 Unrecognized command error or one of its analogues.
- Normal commands produce completely unexpected responses.
One of the most common offenders creating these issues is the Cisco's PIX firewall software, specifically their Mailguard feature. Since it allows only seven basic SMTP commands and no custom Exchange Server verbs to travel in either direction, it has been implicated in creating numerous problems with Exchange server communications. Symantec's Raptor Firewall has also been a culprit.
In short, any firewall or proxy that screens SMTP/POP3 verbs needs to be given a careful once-over when used with Exchange Server.
About the author: Serdar Yegulalp is editor of the Windows Power Users Newsletter.
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Related information from SearchExchange.com:
Expert Advice: Cisco PIX firewall causing Exchange connectivity problems
Tip: Firewall policies and SMTP line lengths
Tip: How HTTP verbs can 'hang' Outlook Web Access
Reference Center: Exchange Server firewall tips and resources
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