mail queue stuck for good in 2.5.1

We upgraded from 2.4 to 2.5.1 od Friday without any problems (well, apart from some stray ERRORs and WARNINGs in the log which began to appear in the error log, but that's another story).

On Monday around noon JIRA stopped sending e-mails without any apparent reason or usage pattern. Flushing the queue in Administration does not work. No e-mails get flushed and nothing is written to the log (we left it at default WARN level).

What is even more frustrating is that scheduler seems to be working. It has "Paused: FALSE, Shutdown: FALSE" and trigger's "Next Fire" is advancing (it got stuck a few times in 2.0.x, but it printed some errors in the log). As usual, sending a test mail through JIRA works.