So, I recently went through an exercise where I archived old mail in my GMail account. My inbox was ENORMOUS, like 80000+ emails. In doing this archiving, it seems that Postbox (an e-mail client based on Thunderbird that costs $29.95. Seems expensive when most e-mail clients are free (come with your OS), or webmail systems do what most people need. I got into Postbox early on, so I stuck with 'em. Anyway, Postbox wouldn' t get out of a Not Responding state after I did this archive, probably because the inbox was out of sync with what was on the server.
Nevertheless, I butchered my profile on the machine, at which point, Postbox wouldn't even LAUNCH, saying that Postbox was already running. It seems that you shouldn't go into your AppData/Roaming folder and delete your profile folder. Rather, you should use the Profile Manager to make these changes. So, I went into Postbox's Profile Manager (same as Thunderbird) and deleted the default profile and created a new one. Right after that, Postbox didn't complain about running already and all was well in the world of Peter's e-mail.