Users are getting mail sent to "INVALID_ADDRESS@.SYNTAX.ERROR" or "MISSING_MAILBOX_TERMINATOR@.SYNTAX-ERROR.".
Addresses in the headers such as the above are produced by PHP, not IMP (via the IMAP c-client library), and are most often the result of a user using semicolons instead of commas to separate addresses, such as is done in MS Outlook Express. Discussions on the mailing list tend to conclude that the solution to this is "well, don't do that, then", since both commas and semicolons have their own meanings in the Internet message format standard, RFC 822.