Failed to connect. Reason No CA

Dear Sirs, 

I tried to sync my account from my old to my new server. I did the below
~.offlineimaprc 

[general]
# List of accounts to be synced, separated by a comma.
# Possible interface choices are: quiet, basic, syslog, ttyui,
blinkenlights, machineui
ui = ttyui
accounts = main 

[Account main]
# Identifier for the local repository; e.g. the maildir to be synced via
IMAP.
localrepository = main-local
# Identifier for the remote repository; i.e. the actual IMAP, usually
non-local.
remoterepository = main-remote 

[Repository main-local]
# OfflineIMAP supports Maildir, GmailMaildir, and IMAP for local
repositories.
type = Maildir
# Where should the mail be placed?
localfolders = /var/vmail/vmail1/openforall.com/mikek/OLD_MAILS 

[Repository main-remote]
# Remote repos can be IMAP or Gmail, the latter being a preconfigured
IMAP.
type = IMAP
remotehost = mail.openforall.net
remoteuser = [email protected]
expunge = no 

then inside the ~ directory I type: offlineimap --dry-run --info 

and I got: 
OfflineIMAP 7.2.3
Licensed under the GNU GPL v2 or any later version (with an OpenSSL
exception)
imaplib2 v2.57 (system), Python v2.7.18rc1, OpenSSL 1.1.1f 31 Mar 2020
imaplib2: 2.57 (system)
Remote repository 'main-remote': type 'IMAP'
Host: mail.openforall.net Port: None SSL: True
Establishing connection to mail.openforall.net:993 (main-remote)
Failed to connect. Reason No CA certificates and no server fingerprints
configured. You must configure at least something, otherwise having SSL
helps nothing.
Establishing connection to mail.openforall.net:993 (main-remote)
Traceback (most recent call last):
File "/usr/bin/offlineimap", line 22, in <module>
oi.run()
File "/usr/share/offlineimap/offlineimap/init.py", line 91, in run
self.__serverdiagnostics(options)
File "/usr/share/offlineimap/offlineimap/init.py", line 523, in
__serverdiagnostics
account.serverdiagnostics()
File "/usr/share/offlineimap/offlineimap/accounts.py", line 194, in
serverdiagnostics
self.ui.serverdiagnostics(remote_repo, 'Remote')
File "/usr/share/offlineimap/offlineimap/ui/UIBase.py", line 473, in
serverdiagnostics
folders = repository.getfolders()
File "/usr/share/offlineimap/offlineimap/repository/IMAP.py", line 452,
in getfolders
imapobj = self.imapserver.acquireconnection()
File "/usr/share/offlineimap/offlineimap/imapserver.py", line 572, in
acquireconnection
af=self.af,
File "/usr/share/offlineimap/offlineimap/imaplibutil.py", line 194, in
__init__
super(WrappedIMAP4_SSL, self).__init__(*args, **kwargs)
File "/usr/lib/python2.7/dist-packages/imaplib2.py", line 2191, in
__init__
IMAP4.__init__(self, host, port, debug, debug_file, identifier, timeout,
debug_buf_lvl)
File "/usr/lib/python2.7/dist-packages/imaplib2.py", line 360, in
__init__
self.open(host, port)
File "/usr/share/offlineimap/offlineimap/imaplibutil.py", line 201, in
open
"having SSL helps nothing.", OfflineImapError.ERROR.REPO)
offlineimap.error.OfflineImapError: No CA certificates and no server
fingerprints configured. You must configure at least something,
otherwise having SSL helps nothing. 

What I need to do? 

Thanks

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