I rewrote the comments, and I changed the name of the test helper class from GmailHandler to BracketFlagHandler, since this is not gmail specific.  (There was a review comment about that that was missed, I guess).

I've applied this only to 3.6 because there is a backward compatibility concern, as I mention in the revised comments: if a server includes a ']' in the text portion, the new regex will parse it incorrectly.  (Which is why the rfc restriction exists in the first place.)  I'm hoping that this is very unlikely, but because it would be a nasty bug if it happens, I only feel comfortable applying it to 3.6.

Thanks Lita, and everyone who reviewed.