Haha I have the same experience... I have given up trying to send emails to t-online, but every other email-provider accept emails from the server I manage. It

Haha I have the same experience... I have given up trying to send emails to t-online, but every other email-provider accept emails from the server I manage. It sends a few thousand emails per day. A few years ago we had problems, but then I realized some of the emails sent from our servers had non-ascii characters in headers (subject, from, to) which caused email-providers to distrust our server. Using encoded-words syntax ("=?UTF-8?B?" + BASE64(text) + "?=") fixed that problem: https://en.wikipedia.org/wiki/MIME#Encoded-Word https://en.wikipedia.org/wiki/MIME#Encoded-Word