OT: Ideas as to best way to do this

[Koopmann, Jan-Peter]

> How do they do it then?

http://www.snertsoft.com/smtp/smtpf/smtpf-cf.html#smtpf_grey_list

By taking the PTR record and removing the first label. So greylisting
considers mail1.google.com the same server as mail2.google.com.

Regards,
 JP