So the bad case is: 1. mails are tiny 2. RAM cache is big 3. cache doesn't get flushed to disk for 1000s of mails 4. crash happens seldomly, but severely :) -> 1000s of mail lost by the crashNow I see :)
:). So fsync/fsyncdata takes care of those rare crashes which is why mail servers should rarely lose mail. I say should because the journal mode available/used affects fsync/fsyncdata too.