With the imapx backend and both incoming and outgoing connections set to TLS encryption, my password gets sent in plaintext (over a non-SSL/TLS connection) to the IMAP server at the time I send an email. I'm not sure why it's opening an IMAP connection at that point. I've confirmed that it's sent in plaintext by running dsniff on the appropriate interface.

This problem doesn't happen with the regular IMAP backend. And a colleague (using the same server) cannot reproduce the problem with Evolution/EDS 3.0.2-0ubuntu1~natty1 (I'm using 3.0.0-1 from Debian experimental).

This bug seems similar to another bug that was fixed quite a while ago: bug #350617

Symptoms:

Error message after mail is sent
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Your message was sent, but an error occurred during post-processing.

The reported error was "Failed to append to treitter@<mail server>:Sent: No output stream
Appending to local 'Sent' folder instead.".

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Then Evolution keeps asking me for my IMAP password. Entering even my correct password results in the following password prompt continuously until I close it:
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Unable to authenticate to IMAP server.
IMAP command failed:[CLIENTBUG] Plaintext authentication disallowed on non-secure (SSL/TLS) connections.
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It looks like my mail server is doing the right thing in rejecting this plaintext password over an insecure connection.