Exactly what does that mean? I have a feeling that it will mean a domain registray you use is changing their provider so you domain will be moving to a new regstrar. If this is the case, what will be happening to the mail on the mail server. Some migrations move the mail, others delete the mail. From Thunderbird's perspective connecting to say mail.yourdomain.com. There may be a disruption caused by DNS issues, but that is something your provider should have informed you off. You might have to update the password after the migration. Otherwise the change should be transparent as the server names ports and connection security are unlikely to change. (they should not normally if the connections are set up in a standard way using standard connection encryption. If you use IMAP and mail is not migrated as part of the move, the mail will eventually be lost. Use the import export tools to export the mail from Thunderbird as mbox files, then use the same tool to import them to local folders. That breaks the IMAP links and makes the mail only reside in your local computer. Remember you will be responsible if you loose it after it is only on your local machine. https://addons.thunderbird.net/en-US/thunderbird/addon/importexporttools/