Not receiving some messages.

I have a gmail that forwards to my personal mail domain on mailcow. But some mails aren't getting through. It's currently 3 senders that has this issue. All other senders gets forwarded just fine.

Gmail links this error message to one of the senders: "This message has been blocked. Reply from server: smtp; 554 5.7.1 This message does not meet our delivery requirements".

I've had one of the senders try to send directly to my personal domain and it's still not getting through. Other senders have no issue sending directly to my personal domain.

I'm really scratching my head on this one. And I'll be honest, I don't know if it's a mailcow issue or a DNS issue. I have spf, dmarc and dkim setup and it checks out fine in mxtoolbox.