Yes so to migrate them all to gmail. I will have to first setup mailbox in Linux. Configure in thunderbird. Configure IMAP in gmail and then copy-paste! 100s of e-mails everyday. (I think gmail has limitation of X number of e-mails per day)
Twitter gives you a similar export file. BUT the difference is that their edition is a fully functional web app and has JSON files as well. You can even search through your database using their exported data. In.com? This is the worst possible way of offering data export.