I'm Back!

Much thanks to Bob Clingan, who pointed out a seemingly unrelated
setting in GMail, my email woes seem to have vanished.  The
culprit was choosing UTF-8 as my outgoing mail encoding, rather than
'default' (which is ISO-8859-1 for me).  Of course, the charset
has nothing to do with the problem, but Google decided UTF-8 messages
should be sent base-64 encoded, while 'default' messages go in plain
text.  No idea why, but fortunately my issue seems to be solved,
as long as I don't need to send any non-latin-1 characters in my email.

Thanks
to everyone who helped me in troubleshooting the issue over the past
few days, and especially to Bob who came up with solution (even though
it was backwards).