@Platon42
Looks like you're the one with the email issue. Check your SPF record.
Received-SPF: pass (mx-c: domain of chrisly*AT*platon42.de designates 87.230.87.142 as permitted sender)
Looks good to me. And it's nothing I changed over the last couple of years. And it's my first bounce in the last 20 months or so... if the SPF record is broken, I would have assumed a LOT more bounces.