Hi, I just spent nearly 3-4 hours trying to get GML to send the messages properly. It seems this has its flaws: why doesn't it retry sending the erroneous messages? Why doesn't it have a halt button? I guess I'll drop a mail to the author.
Anyways, the problem was trying to add aliases for easy labelling. For instance, mymail+whateveralias @ gmail.com (without spaces). While using the alias function, there was no way the mail could get to the inbox. Go figure. I tried with/without quotes, with the version 0.5 (gui) and with the version 0.4 (command line), with strict mbox or less-restrictive mbox interpreter, to no avail.
So I ended up frustrated, and just selected less than 10 emails I would keep, and forwarded them (the barbarian way) to my gmail account. I hope gmail implements mbox importing one of these days.