Yes, Gmail only accepts SSL connections to its IMAP and POP3 services.
If the mail client doesn't support SSL, a workaround might be to install stunnel (if you can get it to run on Haiku, or have a spare Linux/Windows box), then add a service like this, to stunnel.conf
:
[gmail-imap] accept = 143 client = yes connect = imap.gmail.com:993
Then set up MDR to connect to localhost:143
without SSL. (Of course, replace localhost
with the address of your stunnel
server, if that's different.)