Simply set up your account as IMAP on each computer.

Thanks for your rapid response. However, I failed to mention one of my accounts is POP and does not support IMAP, so your elegant solution does not solve my problem. I suspect I need a messier solution.

You can indeed use Dropbox or similar and yes it is messy.

First concern, for me, is that many of my mailstore files are bigger than my Dropbox allowance.

Second concern is that Thunderbird's default storage mode is to place all the messages in a folder into one file, so adding a relatively small email message forces the upload of a large file. If you're prepared to use the experimental maildir format, you can make it store each message in its own file, avoiding the massive upload/download issue.

Third concern is that while a mailstore can be moved, it's down to you to keep track of it, since it becomes detached from the regular profile. You might be able to do something clever with softlinks or symlinks.

Each account has a box labelled "Local Directory" which will tell you where the messages are stored. Use that to identify your mail store, then move or copy the files to Dropbox, then amend the Local Directory setting to point at the Dropbox folder.

Note that it is, in a way, "all or nothing". You can move an entire account's mailstore, but not an individual folder.

Another approach is to use an IMAP connected account and have all the POP traffic directed to it. Many email providers will allow you to set up an email address from a different provider using an alias. Gmail and GMX both offer this feature.

Thanks. Very helpful.

You have given me much to think about. DropBox space is not an issue. I was unaware that messages in a folder are in a single file.

I thinks I will look into your alternate approach as one of my accounts is Gmail.

Gabriel2 said

Thanks for your rapid response. However, I failed to mention one of my accounts is POP and does not support IMAP, so your elegant solution does not solve my problem. I suspect I need a messier solution.

Create an account with a provider that does support IMAP. Add you pop account to the new IMAP one and have the provider check your POP account for you. Google, Yahoo and Outlook all do this.

Use the imap account and configure it to send mail using your pop account SMTP server. That way your POP account is still active, everyone can still sue it and you get to use it as IMAP.

I like this much better that having mail stores on remote cloud servers. I see that as asking for trouble