Mail client connection issues

Firstly we have RoundCube set-up and this is all working great for both sending and receiving of email however when trying to use a mail client we seem to have connection/authentication issues and timeouts.

We have a valid Let's Encrypt certificate and have set this to also be used for postfix/dovecot and we have mail.domain, smtp.domain etc included in the certificate

Have tried authenticating with SSL/TLS on and also off however just timing out, can anyone advise what is wrong/how to troubleshoot this issue?

Thanks in advance!