Emails are not delivered through email clients like Thunderbird and Outlook with SSL/TLS. Can't send/receive. However webmail works fine.
Misconfiguration of config file
/etc/courier-imap/imapd-ssl. Mail clients are not unable to connect through TLS (latest version).
Comment out the following line from /etc/courier-imap/imapd-ssl file to fix the issue:
Check if the connection works through TLS:
# openssl s_client -connect imap.example.com:993 -tls1 CONNECTED(00000003) depth=3 C = SE, O = AddTrust AB, OU = AddTrust External TTP Network, CN = AddTrust External CA Root verify return:1 depth=2 C = GB, ST = Greater Manchester, L = Salford, O = COMODO CA Limited, CN = COMODO RSA Certification Authority verify return:1 depth=1 C = GB, ST = Greater Manchester, L = Salford, O = COMODO CA Limited, CN = COMODO RSA Domain Validation Secure Server CA verify return:1 depth=0 OU = Domain Control Validated, OU = PositiveSSL Wildcard, CN = *.example.com verify return:1 Certificate chain 0 s:/OU=Domain Control Validated/OU=PositiveSSL Wildcard/CN=*.example.com i:/C=GB/ST=Greater Manchester/ ... ...