Hotmail is one of the biggest nightmares you can think of. They provide a form to unblock your IPv4 address and also to apply for their anti spam programs, but this won't help if you have a small mail server with < 100 mail a day. Believe me, I tried.
So you have setup your own mailserver with Dovecot and Postfix and it's working fine (DKIM, SPF etc), except for sending mails to Hotmail and Gmail. The problem: Sending mail to Hotmail/Gmail Guess what, you are not the exception, many people with self small hosted mailservers complain about Gmail
Nein die sind einfach sehr restiktiv. Kann man zwar umgehen indem man die Mails über ein Relay schickt, aber das geht bei mir am Sinn der Sache so wenig Metadaten wie möglich zu hinterlassen vorbei.