Microsoft public email servers (e.g. hotmail.co.uk/live.co.uk/outlook.com) seem to assume that any new SMTP server on the internet is a spammer unless you tell it otherwise. If you get lots of emails bouncing back from those domains (and similar), but nowhere else, that look like this:
This is an automatically generated Delivery Status Notification.
Delivery to the following recipients failed.
Diagnostic-Code: smtp;550 SC-001 (COL004-MC5F36) Unfortunately, messages from 18.104.22.168 weren’t sent. Please contact your Internet service provider since part of their network is on our block list. You can also refer your provider to http://mail.live.com/mail/troubleshooting.aspx#errors.
then go to this link, and fill in the form. All being well you’ll get a ‘OK, we trust you’ email in a few hours and things should stop bouncing.