Why do I get this: 452 4.5.3 Recipients belong to multiple regions ATTR38

Hi,

I moderate a mailing list of some 100 gmail, private and various Microsoft hosted addresses.  When the list sends out a message there are 5 addresses that respond with this message.  They are all hotmail addresses, none of the others do this.  The full message is:

host hotmail-com.olc.protection.outlook.com [104.47.4.33]
    SMTP error from remote mail server after RCPT TO:<*** Email address is removed for privacy ***>:
    452 4.5.3 Recipients belong to multiple regions ATTR38 [AM5EUR02FT030.eop-EUR02.prod.protection.outlook.com]:
    retry timeout exceeded

These addresses are a subset of the microsoft addresses, they aren't even all the hotmail addresses on the list.

One of the addresses is my own test address, that has never marked anything as spam, and only receives email from this address.  Also the mailing list is whitelisted in my test mail account. So it isn't (only) that the user has bounced previous mails from this source.

The most recent email is a very simple text only email. 

I have DMARC and DKIM set up. Prior to this Gmail used to bounce as well.  But for the last ~12 months only these hotmail addresses have bounced.

Thanks

Crispy