Dovecot/Postfix Troubleshooting on CentOS 7 Server

I am facing issues with my installation of Dovecot/Postfix on my CentOS 7 server. While I can access my email through SquirrelMail using the usernames and passwords I created, I am unable to log in using the fully qualified domain name (FQDN). I would like to be able to send and receive emails externally.

Ideal Skills:
- Proficient in Dovecot/Postfix
- Experienced with CentOS 7
- Knowledgeable in configuring email servers