I haven't been having any problems since the ones that prompted me to start this thread, and which were solved as explained in comment 9...

FWIW, I don't have the "Secure authentication" checkbox marked (I think it doesn't actually matter for gmail, authentication happens after the SSL connection is created?); and (unrelated) I *do* use two-factor authentication, with an application-specific password for modest.

My account is set up in modest with gmail as the provider (as opposed to a generic account with the gmail setting manually created), so choosing port 993, for example, is grayed out. Are you using a generic account which just happens to have the gmail settings, or is the account actually set up with gmail as the provider?

(I don't know of any of this matters, just trying to figure out what could be different in our setups...).

How about outgoing email via Gmail (smtp.gmail.com) -- is that working for you? If not, how is it failing?