We had an issue with a customer whose computer was infected with a virus on February 12th. That was resolved the same day however in that time the virus tried to send itself to honeypot addresses on the SORBS real-time black hole list and so got us on that list.
I went through the de-listing process four times and was told by their support people, four times, that we had been de-listed but each time I check they still show us via their web lookup however mxtoolbox showed it clean so just to be safe I decided to move the mail server to a new IP address.
I also found out barracudanetworks firewall was blocking us based upon that SORBS list entry and they did not find it active so agreed to remove the blacklist.
Still Yahoo.com, Cox.net, MSN.com, and Hotmail.com still are rejecting our mail with a 450 temporary refused error code and a message that says due to spam complaints our mail is being “temporarily deferred”.
I have installed SPF, DKIM, and DMARC on our mail server as well as the clamav anti-virus milter to prevent viruses from propagating themselves. I have tested these using public testing facilities.
Other than wait for whatever database these companies are using to timeout I don’t know what to do since their support people are basically worthless saying that what their spam filtering is based upon is proprietary. Yea it obviously works really well. So that’s what’s going on with those domains.
If you are a customer of one of these companies, Yahoo, Cox, MSN, or Hotmail, please complain directly to their support group.