I have server1 (http://bit.ly/1GjQn68) running Ubuntu 14.04 using the Perfect Server guide with a mail server that works with no problems. I have server2 (http://bit.ly/1HL5ZNx) running Debian 7 using the Perfect Server guide with a mail server that doesn't work. From server1, under Mail (ISPConfig 3), I created an entry for server2's domain and created 3 e-mail accounts. Under server2's DNS records, I've created A record: mail.server2.com pointed at server1's IP address and an MX record for "server2.com - mail.server2.com". From server1, I can send e-mails within the server but can't receive external e-mails. Have I done anything wrong? If I haven't, can you suggest methods to diagnose this?