There are several unit test failures that show up when I run with a build that defaults pluggable stores to maildir. I'm going to put the patches that fix those issues in this bug..
Created attachment 634212 [details] [diff] [review] fix maildir store token for fake headers - checked in. The pluggable stores set the store token when you call GetNewMsgOutputStream, so the imap mail folder object shouldn't do so. The store also sets the message offset. The imap mail folder code was basically assuming berkeley mailbox, which doesn't work with maildir. The removal of the srcKeyArray and srcMsgs is an unrelated change, but those variables aren't used.
maildir store token patch checked in - http://hg.mozilla.org/comm-central/rev/8c820d9d2612
Patch has been backed out in Bug 786595 due to incorrect message body being displayed in certain circumstances.