Comment 17 for bug 689453

You could try toggling
mail.server.default.use_compress_deflate to see if it makes the hang go away, though if it's random and hard to reproduce, that may not be that helpful. How random is it, in the sense of how frequent is the hang? It's unfortunate that turning on protocol logging seems to make this problem go away because my first suggestion would be to try to turn on other sorts of logging, but that may not be helpful.