When a client connects to port 143 and is configured to issue a STARTTLS command, Guam responds with a CAPABILITY response that is the configured backend's response either after implicit SSL (doesn't contain STARTTLS) or explicit STARTTLS (doesn't contain STARTTLS either).
The client however requires a CAPABILITY response with STARTTLS (probably a minimal CAPABILITY response).
Response from vanilla IMAP server:
$ imtest -t "" -u vanmeeuwen -a vanmeeuwen kolab.kolabsys.com S: * OK [CAPABILITY IMAP4rev1 LITERAL+ ID ENABLE STARTTLS LOGINDISABLED] kolab02.kolabsys.com Cyrus IMAP 2.5.5.5-Kolab-2.5.5-5.1.el6.kolab_14 server ready C: S01 STARTTLS S: S01 OK Begin TLS negotiation now verify error:num=19:self signed certificate in certificate chain TLS connection established: TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits) C: C01 CAPABILITY S: * CAPABILITY IMAP4rev1 LITERAL+ ID ENABLE ACL RIGHTS=kxten QUOTA MAILBOX-REFERRALS NAMESPACE UIDPLUS NO_ATOMIC_RENAME UNSELECT CHILDREN MULTIAPPEND BINARY CATENATE CONDSTORE ESEARCH SORT SORT=MODSEQ SORT=DISPLAY SORT=UID THREAD=ORDEREDSUBJECT THREAD=REFERENCES ANNOTATEMORE ANNOTATE-EXPERIMENT-1 LIST-EXTENDED LIST-STATUS LIST-MYRIGHTS WITHIN QRESYNC SCAN XLIST XMOVE MOVE SPECIAL-USE CREATE-SPECIAL-USE URLAUTH URLAUTH=BINARY X-NETSCAPE AUTH=PLAIN AUTH=LOGIN SASL-IR COMPRESS=DEFLATE X-QUOTA=STORAGE X-QUOTA=MESSAGE X-QUOTA=X-ANNOTATION-STORAGE X-QUOTA=X-NUM-FOLDERS IDLE S: C01 OK Completed Please enter your password:
Response from Guam
$ imtest -t "" -u vanmeeuwen -a vanmeeuwen 172.17.42.1 S: * OK [CAPABILITY IMAP4rev1 LITERAL+ ID ENABLE ACL RIGHTS=kxten QUOTA MAILBOX-REFERRALS NAMESPACE UIDPLUS NO_ATOMIC_RENAME UNSELECT CHILDREN MULTIAPPEND BINARY CATENATE CONDSTORE ESEARCH SORT SORT=MODSEQ SORT=DISPLAY SORT=UID THREAD=ORDEREDSUBJECT THREAD=REFERENCES ANNOTATEMORE ANNOTATE-EXPERIMENT-1 LIST-EXTENDED LIST-STATUS LIST-MYRIGHTS WITHIN QRESYNC SCAN XLIST XMOVE MOVE SPECIAL-USE CREATE-SPECIAL-USE URLAUTH URLAUTH=BINARY X-NETSCAPE AUTH=PLAIN AUTH=LOGIN SASL-IR COMPRESS=DEFLATE X-QUOTA=STORAGE X-QUOTA=MESSAGE X-QUOTA=X-ANNOTATION-STORAGE X-QUOTA=X-NUM-FOLDERS IDLE] server ready failure: STARTTLS not supported by the server!