I have one domain configured on a droplet and I want to setup an independent mail server on other droplet with mail.mydomain.com. I have installed the setup as per https://www.digitalocean.com/community/tutorials/how-to-set-up-a-postfix-e-mail-server-with-dovecot
but I can send the emails only, I am not able to setup roundcube on my server and neither able to connect to my mail server from any mail client.

Please help me anybody.