T-Mobile.de has implemented a very strict new email policy, which regulates if an email gateway can sent to them.
The way an email sender can check if they are affected is very easy. A new SMTP error like the following will show up in the mail or SMTP logs of your outbound gateways :
554 IP=xx.xx.xx.xxx - Dialup/transient IP not allowed. Use a mailgateway or contact toda@rx.t-online.de if obsolete. (DIAL)
What must be done to be allowed to send email successfully to t-mobile.de or domains hosted by t-mobile.de ?
a) SPF - Your SPF domain (also called the Return-Path domain) must be example.com or a subdomain of example.com.
b) DKIM - Your DKIM domain must be example.com or a subdomain of example.com.
c) PTR - Your sending IP's rDNS must be example.com or a subdomain of example.com.
And now the biggest constraint. You CAN NOT use shared IP addresses any longer to successfully deliver emails to t-online.de.
