I'm in the process of moving my gmail account to fastmail. Trying to move 250k mail (7 gb) from one provider to another is painful. I'm using the imapcopy.pl script which works by reading one mail from the source account and then uploading it to the target. Apparently it's a pretty expensive process as I'm getting my access throttled both by gmail and fastmail. Gmail even shuts of imap access entirely if you hammer their servers hard enough. Well if they didn't want me to flood them, they should have made it possible to download your mail archive in a single tarball...

So far, after running imapcopy.pl for almost a whole week, only 100k mails have been successfully copied.