Each email address is unique, so a single address cannot access two separate accounts. If you are moving an account from one location to another, then you need to investigate what can be done to rescue the data from the old and move it over to the new. Few email providers seem to be prepared to help in this kind of matter. :-( DON'T fiddle with old account settings. The old and new accounts may not be interchangeable (POP vs IMAP?) and adjusting server settings is the fastest way to lose sight of old messages. Set the account up a second time. File|New|Existing Mail Account... If your old data is stored on your own computer, you don't have any big problems. The old folder will remain visible, though for tidiness, you might want to move old correspondence into the new account or into somewhere neutral such as the Local Folders account in Thunderbird. If it's on the previous provider's server, you need to retrieve it while you can. Again, Local Folders is good for this type of task. As an aside, I think you could do a lot better than yahoo. They don't have a happy history with Thunderbird users in particular, and I believe the issues we have seen here in TB have been echoed in other email clients too.