No, the outgoing mail server from the pool is Amazon AWS and has been for 6 months.
My last email from kano.is was from May, but I did notice there was no SPF validation. You could at least put that in your DNS (if you haven't already done so) if your outbound SMTP server never changes. Spam filters tend to treat SPF-validated emails less harshly. More importantly we have more confidence that it is not spoofed.
The domain in question blocked the first (new) email completely then 90s later replied with a greylist message to the next email.
It repeated the greylist message every 10 minutes after that for a few hours until I deleted the message from the queue.
9 hours later the reverse occurred - greylist first then block half an hour later.
Sendmail works that way, resending, by default ... ... ... so any stupid email server on the internet not happy with how sendmail works, obviously doesn't want people to send email to the server
The server in question includes a web link (and long web page) in the reply that is useless.
But if some server out there thinks than every domain that sends them email needs to contact them first ... well ... that's their mail server's problem not mine
I posted here last time email issues came up, and then moved to Amazon AWS ... so yeah it's rare.