I just tried to send an email to a hotmail address and got this?
hotmail-com.olc.protection.outlook.com[XXX] said: 550 5.7.1 Unfortunately, messages from [XXX] weren't sent. Please contact your Internet service provider since part of their network is on our block list (AS3140). You can also refer your provider to http://mail.live.com/mail/troubleshooting.aspx#errors. [SN1NAM02FT052.eop-nam02.prod.protection.outlook.com] (in reply to MAIL FROM command)
I signed up to snds program by microsoft, and it does seem like that address is in their block list. But I did manage to send an email to hotmail a week ago.
There’s only been a few test emails sent by this host, and no other users. I guess it’s some other digital ocean spammers on this address range? Is this common?
All the various tests I could do for email passed, and sending to other hosts report no issues.