Is your mail server behind ClearOS or in a different location with a different WAN IP?

If it is behind ClearOS then you need to port forward to it. At a minimum, to receive e-mails if people send to you directly (so you don't use app-mail-retrieval) you need to forward tcp:25. If you want to access your mails directly when working externally you will want some of the following ports:
tcp:465 - if your e-mail clients send using SMTPS (recommended)
tco:587 - if your e-mail clients send using STARTTLS (need to tweak ClearOS)
tcp:110 - if you e-mail clients pick up e-mails using POP3
tcp:995 - if you e-mail clients pick up e-mails using POP3
tcp:143 - if you e-mail clients pick up e-mails using IMAP
tcp:993 - if you e-mail clients pick up e-mails using IMAPS

My preference for picking up e-mails is IMAPS

If you are hosting your e-mail server externally then it is your DNS records which need adjusting and your MX record will have to point to your external server to send and receive e-mails externally. To pick up e-mails with an e-mail client, you'll want a separate A record or CNAME record pointing to your mailserver and you'll need to reconfigure your e-mail clients to use that FQDN.