> Just because you send mail to Yahoo through those IPs doesn't mean they > send mail to you from those IPs. It's not unheard of for incoming and > outgoing mail to go through different servers once you get to a certain > size.
Exactly. Some institutions even delegate both incomming and outgoing mail to third parties. $ host ucalgary.ca ucalgary.ca has address 136.159.96.25 ucalgary.ca mail is handled by 5 ucalgary-ca.mail.protection.outlook.com. Which IP addresses the mail from such domains will come from you will only know once it comes, gets greylisted, and stays greylisted for some time, as the resend happens from another IP.
