Hello, you have used wrong subdomain in your example:
root@a10-52-148-181:~# /usr/local/psa/bin/dns --add mydomain.com -srv "" -srv-priority 5 -srv-weight 50 -srv-port 143 -srv-protocol tcp -srv-service imap -srv-target-host imap.mydomain.com
SUCCESS: Creation of DNS record in Domain...