Bugzilla

(In reply to chris.fogel from comment #5)
> These items did not seem to fix the issue. Certain new emails from zendesk come in and still get the <?> subject line.... 
> If I either a) repair folder again or b) edit the raw file and delete the carriage return in the subject line, it corrects just the ones already arrived.
> But the next email that comes in from that source, just <?> again...

That sounds like something different to Bug 1872849 - it seems to me that the mbox file is actually OK in this case, given that you can use "repair folder" to fix it.
From what I can tell, the incoming emails are valid. It's OK for headers to be broken up over multiple lines, as long as the continued lines begin with whitespace (which does seem to be the case here, looking at bug 1888790 comment #2).

Bug 1872849 seems to be more to do with message writes to the mbox file being truncated or interleaved, which "repair folder" would not help.

My suspicion would be that the problem is in the header parsing code rather than the mbox storage code.