General effect that this has Companies doing. *OH* you have an AOL address... Sorry we don't allow e-mail from AOL, which has historically BAD SPAM mailings... either with thousands of recipients or just plain sheer number of annoyances.
I can say, companies such as Meijer, Gordon Food Services, [Sc]Amway all use RBL and MAPS and score appropriately based on them. Those three corporations together have ~200K e-mail addresses and have been hit badly in the past... I personally have help Gordon Food Services and Meijer fix the open-relays they have had in the past... Both of them were misconfigured Checkpoint Firewalls...
But NOOOOO>>>> the Network Admins *KNEW* what they are doing HAD TO BE the Mailservers...
So I captured Packets on *MY* end and they capture on the mailserver... I provided them with a TIME server we all could sync to... therefore they could prove the mail-servers were at fault...
I setup to use thier IP address to relay mail... actually sent *THEM* e-mail at the same time to make them think they CAUGHT IT... well umm... sorry... The Checkpoint smtp server answered the WHOLE conversation... and forwarded the e-mail on... rather than do a port-forward to the REAL servers... After all was said and done, I ended up teaching "Senior Networking Veteran Administrators" alot about Checkpoint Firewalls and how to use them... it's all in the Order... and READ about the options iffn you aren't sure. They thought "allow forwarding/relaying" mail only meant for *THIER* stuff... Wrongo... Now they understand why it's a good thing to use portforwarding and DNAT (Destination Network Address Translation) and SNAT (Source Network Address Translation)...
Best part was... one Corporation refer'd the other to me... to help. Same EXACT problem... Know-It-All "Network Admins" and a Checkpoint Firewall... being misconfigured...