Why is this a problem? It's by design. This is IMAP at work, which lets each of your devices see the same central repository of email messages, on your provider's server. Delete a message and it's gone, removed from the server so it vanishes from all the other devices. If you want to take messages out of circulation, but want to keep them for future reference, I suggest you move them to a folder under the Local Folders account in Thunderbird. Android devices are usually quite selective about what email they download, so you don't necessarily have a complete copy of everything stored on the server also in your phone or tablet. Note the delay when you access an older message or folder that you haven't looked at for some time. It is almost certainly being downloaded on demand. The alternative is to use POP, where each device's email client makes its own local private copy. It will also delete the original from the server, unless your email client offers you a setting to leave a copy. So, with care, you could have a local copy on a phone or table and deleting it could be arranged not to affect the server. I say "with care" because Thunderbird's default behaviour is to delete server copies when the local copy is removed. I can't speak for how other email clients will handle this situation. And some may not support a POP connection; it's not a good fit to the limited memory on portable devices. But, consider; if all of your devices leave messages on the server, how will you stop it just accumulating stuff indefinitely? You'll need to either visit the server's webpage now and then to maintain it, or designate one of your computers as the "master" and use it to delete server copies.