There is one ISP that is rejecting mail from my mail server with the following error: Code: 451 4.1.8 Possibly forged hostname for 18.104.22.168 My domain is mercedesshop.com. Is there a problem with my mail server or is the issue with that ISP? This is the definition I found for error 451 4.1.8: "This means that it was able to resolve the IP address, but it doesn't resolve to a hostname that matches both forward and reverse lookups." Please note that all of my incoming mail is routed through a proxy mail server to filter spam and viruses. I would not think that would make any difference for SMTP.