PHP Mail Not Working

[3DPeruna]

I've got (2) ISPConfig 3 servers. Server A hosts most domains and all emails. Server B hosts 1 domain, but no email. Server A has a direct IP address, Server B is behind a NAT.

I would like to set it up so Server B can send mail (via SMTP) through Server A.

Right now, I'm getting the following on Server B when I try to send mail through Server A: SMTP Error: Could not connect to SMTP host. Neither servers logs show anything (including running tail -f /var/log/syslog on both servers and watching what happens when I hit "send").

Thoughts