"tardamail.com is too new, temporarily blacklisted" is what I see in the mail-logs.
Seems like something I can't do much about at the moment, and should correct itself in a while. It seems to get past most spam-filters though.
I'll consider implementing an email-confirmation backup, thanks!
https://www.linuxbabe.com/mail-server/ip-blacklist-removal
Especially the part about "Rule of Thumb for Running Mail Server on a New IP Address
When you run a mail server on a new IP address, you should not use this IP address to send newsletters (aka marketing emails) right away. Instead, you should use this IP address to send transactional emails for a period of time to improve the reputation.
You may also want to use SMTP relay to send emails for a few days because SMTP relay services have a high IP reputation. After that, send emails directly to the familiar recipients."