From MozillaZine Knowledge Base
- This article was written for Thunderbird but also applies to Mozilla Suite / SeaMonkey (though some menu sequences may differ).
Thunderbird doesn't support converting a POP account to a IMAP account. Its possible to hack the files in the profile to convert the account, but its tricky and time consuming and requires knowledge most users don't have. Its much quicker and safer to create a new account and move the messages to it, and then delete the old account.
- Backup your profile using something like Mozbackup.
- Create a IMAP account using Tools -> Account Settings -> Add Account
- Move the messages from the POP account to the IMAP account, or to folders in the Local Folders directory. Thunderbird 1.5 and later support dragging and dropping a folder to the IMAP account. Otherwise you need to create the destination folder, select all of the messages you want to move (Control-A or Edit -> Select -> All) and then use Message -> Move to move the messages to the remote folder.
- Compact the folders in the POP account using File -> Compact Folders.
- If you have any message filters you can exit Thunderbird and move the "msgFilterRules.dat" file from the directory named after your POP server in the Mail subdirectory in your profile to the directory named after your IMAP mail server in the ImapMail subdirectory. That will let you reuse the message filters. If you used a global inbox the file would be in the Local Folders directory. If this is too confusing just create new message filters.
- Verify the IMAP account is working okay and you didn't forget to move any messages. Then delete the POP account using Tools -> Account Settings -> RemoveAccount.
- Thunderbird doesn't delete the files used in a deleted account. You may want to exit Thunderbird and delete the subdirectory named after the POP server. However, since you emptied and compacted its folders its not wasting a lot of space.