Well, I got this server a while ago and never had any problems sending mails. At least until I tried to send some mail to an t-online customer. It was rejected, because my IP is on a blacklist. As my system never sends any SPAM I assume that the IP was blocked ages ago when it wasn't under my control. So I contacted t-online and suprisingly they replied with some rules they made up: https://postmaster.t-online.de/index.en.html#t4.1
Ok, I have an PTR record which indicates mail usage. The WHOIS information is set by my hosting provider and also met the requirements. But, I don't have a website with contact informations. I don't know why it is needed, especially for a private site and because you can contact the abuse mail my hosting provider has in his WHOIS record, which - as stated in the WHOIS informations - will be forwarded directly to me, but anyway...
Here we go t-online:
For spam/abuse/security issues please contact: