Thanks for the feedback.. in response to your questions..

• I had tried it to some extent, and hadn't seen difficulties. The issue though is that I can imagine it being the kind of thing that'd seem perfectly happy that one time something unforeseen happened and it's under incredible load.. at which point it all goes horribly wrong, data loss and badness.
• It would be logging from all of the concurrent threads to the logfile since, in principle at least, all of the threads constitute the one service and those responsible for administering it would probably prefer a single easy-to-read log.
• I'd not tried putting the lock there since I'd not seen the problem yet, also if Log4perl was happy multithreaded then I'd rather not have the performance hit of another set of locking.
• The seperate functions have unit tests as far as is practical, unfortunately given the way the module slimes under the skin of Sendmail the only full way to test it is to fire off a bunch of mails at it and check it does what's expected. There is a basic test for this too, but it doesn't touch the logging as yet.