A comment by Jan-Lukas Else

So far sending didn't make any problems yet. I just had to contact T-Online.de as they only seem to allow mails from whitelisted servers. On Microsoft's Postmaster tools, I saw that mails will probably go to junk, but I did not send any important mail to Outlook users yet.
But I tried to configure everything correctly: DKIM, SPF, DMARC, MTA-STS, DNSSEC, ...