First, the current version import from outlook is plain broken and has been disabled. If your going to import I suggest you use V17as it is far enough back to miss most of the crashes that occur following letter toolkit changes. Second Outlook must be installed and in a state capable of running the 20GB file. as the importer actually uses DDE to ask Outlook for each mail and contact. My confidence level is somewhat shaken by the fact that this person is having trouble with Outlook, not in Thunderbird ability to suck in all those mails. The next issue is the size of individual folders. While it is technically supposed to be gone, there was a 4Gb per folder limit in the past and I am sure there are still bugs to be found when that limit is exceeded. The limit most certainly exists in Thunderbird 17. So a single huge inbox of 20Gb is almost certain to be a failure, in one way or another. My personal mail store in Thunderbird is close to that 20Gb but whist individual folder sizes are constrained, there appears to be no practical limit to how many folders you can have. Available disk space is probably the closest thing to a limit. This of course assumes POP mail or local folders storage. IMAP is constrained to the size of the server storage made available as a Quota by the server admin.