The ssl and gnutls USE glags eacg enable/disable gnutls now that claws-mail does not use SSL. As a result, if you have USE"=gnutls -ssl", claws is configured with "--enable -gnutls --disable-gnutls", which disable the very feature you were trying to turn on. This patch fixes the problem in the 3.7.0 ebuild, or you could simply drop the ssl USE flag.
Created attachment 178139 [details, diff] Patch for claws-mail-3.7.0 ebuild
Reassigning to / CCing maintainers / net-mail herd.
Right, to comfort users I let USE=ssl do the same as USE=gnutls, but I did not think that people would actually switch off USE=ssl. :) Fixed, but in a different way: I handle it like the USE=imap|nntp dependency with a local configuration variable.