I have an email that's bouncing with an error that says:
smtp; 554 5.7.1 <user@example.com>: Sender address rejected:
example.com is not allowed to send email to MyDomain.com
What's causing this?
This means that the sender's email address (or domain name) is in your Blocked Senders list. To remove the block, login to the control panel and go to Settings > Blocked Senders. From there, click on the checkbox next to the sender's email address (or domain name) and then click on the Delete button.