In the last couple of month we've observed some odd behavior from MM3 (namely the core) where it seems like email that is about to be sent to a large-ish mailing list (10K users) has been processed yet never gets sent out until we restart the core.
I would appreciate any pointers on where to look for the source of the problem. At the moment, since I can't reliably reproduce it I can't really test it nor do I know where to look next time things get stuck. Mailing list in question is high profile thus when event like this happens I have only limited time amount to spend on troubleshooting/digging before I just restart the core (which by the way takes about 45min to fully start... which is another puzzle to solve at later time)
-- Sr System and DevOps Engineer SoM IRT