IMAP proxy for Exchange - encrypted backend Communication?

[tom]

Hello,

I try to setup a IMAP proxy for my old Exchange server.
Running Dovecot v2.x on Centos 7.

So far I follow http://wiki2.dovecot.org/HowTo/ImapcProxy and it seem
to work. The only but major  thing is with this setup - the
communication between proxy and backend is not encrypted. :(

To fix this, I changed the config and add:
imapc_ssl=imaps
imapc_port=993

but it doesnt work, because of verify failure of the self signed
backend certificate:

Jan  5 21:48:55 imap dovecot: imap(user1): Error:
imapc(192.168.1.1:993): Couldn't initialize SSL context: Can't verify
remote server certs without trusted CAs (ssl_client_ca_* settings)
Jan  5 21:48:55 imap dovecot: imap(user1): Error:
imapc(192.168.1.1:993): No SSL context
Jan  5 21:48:55 imap dovecot: imap(user1): Error: imapc: Command
failed: Disconnected from server
Jan  5 21:48:55 imap dovecot: imap(user1): Error: user tkoenig:
Initialization failed: Initializing mail storage from mail_location
setting failed: Mailbox list driver imapc: Failed to access imapc
backend
Jan  5 21:48:55 imap dovecot: imap(user1): Error: Invalid user
settings. Refer to server log for more information.

I didnt found anything in the documentation which tells dovcot not
verify the backend certificate.

Is there a know way to get it runing?

Many thanks for any hint!

regrds,
Tom