Claws uses TLSv1 when connecting to an imap server using Starttls encryption even when the server offers TLSv1.1 and TLSv1.2 This was tested with different servers/providers Tested on an up to date archlinux setup with gnutls 3.4.9-1
That would be a libetpan bug not a claws-mail bug. libetpan is the library claws-mail uses to handle imap connections. Try upgrading libetpan to the latest release, and if that doesn't help, report the issue here: https://github.com/dinhviethoa/libetpan