Claws Mail 3.10 got support for account autoconfiguration:
Auto-configuration of account email servers, based on SRV records, is now possible.
Unfortunately neither the manual nor git log do tell us how to actually use them.
After digging through the source code, I found that Claws implements RFC 6186, "Use of SRV Records for Locating Email Submission/Access Services".
I created SRV DNS records for _submission._tcp.cweiske.de. and _imaps._tcp.cweiske.de., pointing to the mail server and the correct ports. With that, auto configuration worked fine (except for the user name, which is a bug):