I looked at your basic DNS records, seems workable for mail routing (A, MX, PTR), Your open ports look fine, too.
PORT STATE SERVICE
25/tcp open smtp
80/tcp open http
110/tcp open pop3
143/tcp open imap
443/tcp open https
465/tcp open smtps
993/tcp open imaps
995/tcp open pop3s
1723/tcp open pptp
.
So how about some error logs or SMTP error codes?