From MozillaZine Knowledge Base
- This article was written for Thunderbird but also applies to Mozilla Suite / SeaMonkey (though some menu sequences may differ).
If you change email providers don't try to edit the existing account to use a new email provider as that will have bad side effects such as changing where the account stores its folders, meaning you won't be able to see your old mail.
- Copy any messages in the old account that you want to keep to "Local Folders". (If you are using a POP account configured to use a Global Inbox you can skip this step as the messages are already in "Local Folders") Thunderbird doesn't support a menu command to copy folders. However, you can drag and drop any folders that don't already exist wherever you are copying them to. If you do that it also copies any child folders. You can copy the contents of folders such as Inbox and Sent by using Control+A to select all of the messages in the folder and then either dragging and dropping the selected messages in one step, or using "Message -> Copy To" to copy them.
- Verify the messages were copied okay. For example, verify your largest folder has the right number of messages and read one of them.
- Delete all of messages in the old account, empty the trash and compact the accounts folders. The reason for doing this is when you delete an account it doesn't delete the accounts folders.
- If you were using a IMAP account you may have offline folders due to Tools -> Account Settings -> Synchronization & Storage -> "Keep messages for this account on this computer". If you want to keep some of those messages switch to working offline using File -> Offline -> Work Offline (so that they are visible), copy the messages, and then switch back to working online by unchecking File -> Work Offline. You could also use the icon that looks like two adjacent monitors in the lower left corner of the main window to switch between online and offline.
- Delete the old account using Tools -> Account Settings -> Account Actions -> Remove Account.
- Add a new account using Tools -> Account Settings -> Account Actions -> Add Mail Account.
- If you had used the old accounts SMTP server as the default SMTP server, replace it using Tools -> Account Settings -> Outgoing Server (SMTP) -> Set Default.
You could use the ImportExportTools add-on to import the old accounts folders instead of using Local Folders for temporary storage, or merge the contents of the local directory of the two accounts using windows explorer. However, that is more complex and its easier to make a mistake that way.
If you are using global search it may take a while for it to rebuild the search index to use the new location of any messages you saved.
If you run your own IMAP server and switch to a different hosting company Thunderbird is uneffected by your changing the domain registration and switching servers (other than it updating the security certificates). However, if you need to physically copy the contents of the server to a new host do this before redirecting the DNS alias of your domain to the new host and reconnecting using Thunderbird.