Thou Shalt Honor RFC-822, Or Not Read Email At All (08 Nov 2009)

Thunderbird is my preferred mail client, and so I'm using it to connect to our Exchange mail server at work via IMAP. This worked great for quite some time, until Thunderbird started reporting errors like "The requested message could not be converted to an RFC-822 compatible format" whenever I refreshed my inbox. Today, I finally bit the bullet, and started an IMAP debugging session using telnet.

With the excellent tutorial Accessing IMAP email accounts using telnet at hand, getting up to speed with IMAP was a matter of minutes. Digging into the protocol was pretty cool (in my very own sick and geeky sense of coolness), and I learned a few things. It seems that Exchange Server, when contacted via IMAP, tries to convert all messages into RFC-822 format, and when this fails, said error is reported. So as far as I can tell, this isn't really an email client issue, but rather a problem in either the original message data or in the conversion process on the server.

However, those insights didn't solve the problem right away. I tried deleting a few suspect messages by ID, without seeing much of an improvement. After a while, I resorted to a more radical experiment:

  • Create a temp folder on the IMAP server
  • Move all messages from the Inbox to the temp folder
  • With all inbox messages out of the way, hit Thunderbird's "Get Mail" button again.

You'd think that with an empty inbox, there shouldn't be a reason anymore for error messages - but still, Thunderbird kept reporting RFC-822 conversion issues.

Thunderbird glitch? Bad hair day? What was going on?

Desperate as I was, I did something which, being a Thunderbird devotee, I would never do under normal circumstances: I ran Outlook. And indeed, to my surprise, Outlook still displayed a number of messages in my inbox! All of those were meeting invitations. Apparently, those were the messages which were in some way incompliant or at least incompatible with RFC-822. I deleted all those messages in Outlook, emptied the "Deleted Items" and "Trash" folders, then ran Thunderbird again.

Argl. "Get Mail" still reports the same error messages.

Another telnet session followed:

  telnet int-mail.ptc.com 143
  . login cbrod JOSHUA
  . select INBOX
  . fetch 1:100 flags

Surprisingly, IMAP still reported 17 messages, which I then deleted manually as follows:

  . store 1:17 flags \Deleted
  . expunge
  . close

And now, finally, the error message in Thunderbird was gone. Phew.

In hindsight, I should have kept those invitation messages around to find out more about their RFC-822 compliance problem. But I guess there is no shortage of meeting invitations in an Outlook-centric company, and so there will be more specimens available for thorough scrutiny big grin


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