nsIMsgLocalMailFolder.addMessageBatch() is only really used by test code, but it's handy for loading in raw email data.

But it screws up totally on messages that have unescaped "From " lines in the body - the .messageSize attribute on the resulting msgDBHdr is waaaay off.

What it should do is take input email (RFC5322 data) with no leading "From " line and no "From " quoting, and the mailstore should handle all that behind the scenes if required (i.e. only for mbox stores!).

This is a new unit test to illustrate the problem with nsIMsgLocalMailFolder.addMessageBatch().
Once fixed, we could turn this into a proper unit test to exercise addMessageBatch() better.