I think it means of the authentication methods the server supports (GSSAPI NTLM), your client doesn't support any. SASL is a library for authentication often used in for instance SMTP and POP3/IMAP servers, I believe it came forth from the Cyrus IMAP server.

If you can't find any module to support SASL/GSSAPI or SASL/NTLM correctly, you could perhaps send the email without authentication... Or try to pipe it through the local mail server if whatever mail injection binary your MTA has, supports arguments for remote authentication.