Is there a program written for Linux which sync between devices? By sync I mean e.g.

• account (like Mozilla account)
• QR code scan, like Brave browser

There is a 10y+ wishlist feature for Thunderbird to implement Firefox-like cross-device sync, but I assume the feature won't be implemented in the next 10. Other applications I have tried like Kmail and Sylpheed don't/didn't have this feature when I last used them, some 5 years ago. What is the current state?

Well, I don't know how you're doing things, but all my installs of Thunderbird update to the current state of the respective a/c mailboxes whenever I launch them.

To me, this IS 'syncing'. What would YOU call it, hmm..?

Mike.

It all depends on how you have thunderbird (or any other email client) configured.

If the client downloads and deletes the downloaded mail from the server when it connects then:
NO they won't sync between multiple devices/clients because the mail is no longer on the server when the next client connects.

If the client downloads the mail but does not delete it from the server then:
YES it will allow syncing multiple devices/clients.

In the second case mail that is deleted on the client should be deleted on the server when that client next connects.

I assume you are asking just about email and like the previous posts it depends on how you configure Thunderbird. To elaborate on computersavvy's post:

If you are are connecting to your email server using POP3 then all email is first downloaded to your PC and then deleted from the server. This means you can not access the same email from other devices.

If you are connecting to your email server using IMAP then email is only downloaded when you click on it and it is not deleted from the server. This means you can access the same account from multiple devices.

First of all, let me apologize for misphrasing my question. You have essentially explained the difference IMAP vs POP3 and e-mail synchronization options. Data synchronization is not what I meant, but configuration.

What I meant is something akin to a Thunderbird account. Use case: you have a company and it has some standard filters and rules, plugins as well. Current state in Thunderbird: copy/paste the config path, then hope it is parsed correctly on all target platforms. This doesn't work universally, not even reliably between Linux distros. In contrast: you can create e.g. a Mozilla account. When you log in from a different device to this account using Firefox (regardless of platform), you can select what to sync. I.e. you can have saved bookmarks, configuration, plugins etc. everywhere with only 1 login per device.

So, now I rephrase my question: is there a comparable e-mail client?

Evolution has "Back up Evolution Data..." and "Restore Evolution Data..." under the File menu, but I cannot say what exactly gets backed up as I never used it.

To add to what michaelk said…I have two thunderbird accounts set up on one computer (happens to be a Windows box) for a single account on my server. One (set up along time ago) is configured to use POP3 with “leave message on server” checked. I use that one to download new messages to the desktop as a backup. The other TB account is configured to use IMAP.
All my other devices access that account using IMAP. I use TB on my Linux desktop and my MS Surface laptop, Apple Mail on my iPad and iPhone.
As the usage on each device is different I set up each configuration manually, but one could theoretically copy a TB configuration to a different computer and have it work. My original setup of TB on the Linux desktop was done by copying the configuration from the Windows desktop, and then removing the Thunderbird account set up as POP3.

There may be differences in the config location between operating systems and even between distributions, but a given configuration should work in any TB installation.

I'm going to assume that you tested all email clients available to you through your distro's repositories, and none of them offer a "Sync" (as in Firefox Sync) option.
I will further assume that Thunderbird does not offer Sync, and that you searched its Addon catalog without success.

I will further assume that you did not search the web for "thunderbird sync", becasue for me this is the 1st result. It comes down to this:

• lacking a "cloud" (a.k.a other people's computers) solution from TB itself, there are various ways you can implement something like that yourself: dropbox, your own VPS, your own physical server, etc. etc.
• hooking a remote directory tree into your local filesystem is something very commonly done on Linux. For starters, I recommend sshfs.
• a complete sync is possible for TB installs on same/similar operating systems, but fragile
• a partial sync should be possible in more cases