I got if fixed server side disabling GSSAPI auth
before:
SMTP S: 250-AUTH=PLAIN LOGIN GSSAPI
after:
SMTP S: 250-AUTH PLAIN LOGIN
GSSAPI was not well implemented server side so I asked support to disable it
the problem is eM Client won’t let us select authentication method manually (thunderbird does, heck even Outlook does)
hope it helps
Diego