Well I just found this old post here from 2011 before STARTTLS was enabled:
Originally Posted by nars
When I checked mine with telnet I seem to not be supporting it, which explains why it isn't working.
The conf file does have it though. So maybe this is a bug?
* OK [CAPABILITY IMAP4rev1 UIDPLUS CHILDREN NAMESPACE THREAD=ORDEREDSUBJECT THREAD=REFERENCES SORT QUOTA IDLE] Courier-IMAP ready. Copyright 1998-2003 Double Precision, Inc. See COPYING for distribution information.
# Whether or not to implement IMAP STARTTLS extension instead: