Mail server on 2 ISPs cannot receive outside mail

Mail server on 2 ISPs cannot receive outside mail

I have been trying to fix this problem for a week now. My network has 2 ISP:
1: fixed IP -> Billion router -> mail and web servers
2: dynamic IP -> Linksys -> normal web browsing
They all used to work flawlessly until I reset the routers a week ago
(following an advice by ISP to fix a problem). Until now I can make
everything work except receiving mails from outside domains.
Billion - DHCP (192.168.2.1) is turned off. Virtual servers in NAT page
set to corresponding server local IPs.
Linksys - DHCP (192.168.2.2) is set to relay to 192.168.2.101 (I don't
know why. It was set by former admin who quits 2 months ago) Single Port
Forwarding was set like before.
Here is the DNS setting in DNS-DIY:
(blank) - A - (global IP)
* - A - (global IP)
www - A - (global IP)
(blank) - MX - mail
mail - MX - (global IP)
Telnetting the local IP and mail.xxxx.com for 110 and 25 works, but
xxxx.com doesn't.
Another thing I can tell is two routers are connected in some way. I don't
know about details of the mail server. I am just a programmer and
completely new to this network stuff. Even the network admin who quits 2
months ago cannot figure this out.
This issue is critical to the company I am working, so I need this fixed
ASAP. I will provide any more information I can.