You guys, the removal of the "abuse.net" RBL is indeed what was causing all my server's e-mail to be rejected; it didn't matter whether I was trying to send e-mail through a POP3 program, or waiting for mail being sent from outside e-mail accounts .... nothing at all was being allowed in.
I got the following response no matter what I tried to do e-mail-wise, in my POP3 mail program:
The message could not be sent because one of the recipients was rejected by the server. The rejected e-mail address was 'admin@******.net'. Subject 'Test e-mail', Account: '*********', Server: 'mail.*********.net', Protocol: SMTP, Server Response: '451 postmaster@localhost', Port: 25, Secure(SSL): No, Server Error: 451, Error Number: 0x800CCC79
I was literally pulling my hair out!! I have been trying so hard to fix this problem .... I re-built my server (software & hardware), tried different network cards, trying random DNS server combinations, the list went on and on. I came THIS close to ... <gulp> ... having someone in my organization build a .... a .... Exchange Server. Yes, that deperate.
Then I tried, once again, in my almighty quest to come to a possible positive solution, to look around. Little did I know that a simple RBL removal would DO THE TRICK!!! That "abuse.net" being removed instantaneously solved my problems.
I had removed all but the last 3 RBLs on my list (I used every one offered; the server has 512MB RAM on it so I wasn't worried) and was reading posts at the same time. When I removed the abuse.net one, everything worked. Everything. I've been adding a new RBL and restarting the service using the link in the Config Manager, and each time I can send e-mail. I've made my way all the way back to the top of the list again, minus one, and it still works!
You guys, read posts carefully. One thing, in this case, was all it took. You never know how simple a complicated problem's answer can be.