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Microsoft public email servers (e.g. 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.

Final-Recipient: rfc822;

Action: failed

Status: 5.5.0

Diagnostic-Code: smtp;550 SC-001 (COL004-MC5F36) Unfortunately, messages from 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

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.

Here is the Link; it is long and convoluted!