Debian 10 was released 6 July 2019 with a .deb for claws-mail 3.17.3 but it will not run unless libetpan.so.17 is installed from the Debian 9 repository. This appears to be a missed dependency for the .deb. A better solution is to recompile the .deb to use libetpan.so.20 which is part of the Debian 10 release.
The Buster (Debian 10) version of claws-mail package already depends on libetpan20 package, see https://packages.debian.org/buster/claws-mail Just to be sure I've also downloaded a claws-mail_3.17.3-2_amd64.deb from a random Debian mirror and checked that the control file has "libetpan20 (>= 1.9.2)" listed within the dependencies. Not sure why yours requires a different one, there's several possibilities. Reinstalling the package from the Debian repositories (make sure it comes from them) should fix your problem. BTW, for the future: if this is a Debian bug you should be reporting in the Debian BTS (see https://www.debian.org/Bugs/Reporting ), not here :-)
Using synaptic, I did "mark for complete removal" of libetpan17 in a preliminary action. Thereafter, it was not enough for me to "mark for reinstallation" nor to "mark for removal" followed in a separate action to "mark for installation" claws-mail from the Debian 10 repository. I had to save my ~/.claws-mail directory then use "mark for complete removal". After the complete removal action had finished, I installed claws-mail again with synaptic. Claws-mail started without the error and it wanted to do initial setup, so I exited and returned my saved copy of the ~/.claws-mail directory then restarted claws-mail. Claws-mail worked as expected. Conclusion: this is a synaptic related issue and has nothing to do with the claws-mail .deb which I thought might have been prepared by you. Sorry for the noise.
(In reply to comment #2) […] > I had to > save my ~/.claws-mail directory then use "mark for complete removal". After > the complete removal action had finished, I installed claws-mail again with > synaptic. Saving your ~/.claws-mail should not be required, sane packages do not install or remove files in/from $HOME directories. > Conclusion: this is a synaptic related issue and has nothing to do with the > claws-mail .deb which I thought might have been prepared by you. Sounds strange to me, it should have been able to install/upgrade claws-mail without problem, but I do not use synaptic, so you could be right. > Sorry for the noise. No problem, glad it's fixed now :)