From discussion on IRC, I have understood that claws-mail currently does not support Kerberos ticket-based authentication (that is, use the user's TGT to request an imap/ service ticket). Indeed, when I try to force GSSAPI while having a valid krbtgt, claws doesn't appear to try and an fetch imap/ ticket, it just errors out and dovecot says "Aborted login (no auth attempts)". Mutt appears to work fine: it logs in and afterwards klist shows me it now has imap/ auth tokens. But even with those tokens present, claws does not use them. Filing this as wishlist bug. However, the language in Bug1679 seems to suggest that ticket-based logins used to be possible?
I have seen the same problem. I stopped using claws-mail a few years ago because of this bug. I was never able to use claws with GSSAPI, so I can't say whether it once was possible. I tried again today, and the problem is still there. Dovecot is my imap server, and alpine can use my kerberos tgt to get an imap service ticket. When I try to do the same thing with claws, dovecot sees the connection but tells me "Aborted login (no auth attempts)".