Sometimes an SMTP conversation with a remote host will abort with a 501 "Data format error" message. This can happen if the remote host is transferring an e-mail message that has an address with an apostrophe in one of its header fields.

Note that this problem does not refer to an apostrophe used in the friendly name field for an e-mail address, but the e-mail address itself. For instance, a TO: field in an e-mail that reads:

To: Phil L'Honor [phil@company.net]

would be okay.

But, the following would not:

To: Phil L'Honor [phil_l'honor@company.net]

Older e-mail servers that are not entirely compliant with RFC821 (the Internet standard for SMTP messages) will experience this problem more often. In other words, Exchange 2000 or 2003 may not have a problem with an apostrophe character, but another remote host might.

Sometimes the problem does not lie with the e-mail server per se, but with the firewall. Many firewalls are programmed to intelligently parse traffic on certain ports (such as port 80, 25 or 110). Upon seeing the apostrophe character as part of the e-mail address, the firewall may reject it as being non-RFC821 compliant.

If you are having trouble receiving e-mail from a third party that has an apostrophe in the e-mail address, there are a few options on how to handle this. One is to disable the firewall rule that governs RFC821 formatting (if it's available, and

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if it is indeed the reason the message cannot be successfully received), but many administrators may balk at this. Another is to simply write a letter to the postmaster of the other host and let them know that messages from their domain cannot be delivered due to this quirk. If they are responsible about their e-mail, they will remedy the problem by replacing the apostrophe (with an underscore, for instance) or by eliminating it entirely from the e-mail address.

As a general rule, make sure that no e-mail addresses in your own organization contain an apostrophe. One quick way to find out if any of them do would be to export a list to a CSV file and search it, then make changes as needed.


Serdar Yegulalp is the editor of the Windows 2000 Power Users Newsletter.


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This was first published in September 2004

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