An email is added to our mailing suppression list when our infrastructure determines that the address is either invalid (having several attempted -and bounced- sending several messages to the address over time), or problematic. Problematic addresses are often caused by spam having been sent out from a given address (though it is often due to spoofing).
If you have checked the Email Log Report, and it shows successfully sent emails that your users have not received, you can ask Tech Support to check the Suppressions List, which may show how or where your emails are vanishing.
Note that emails that do not arrive at their destination are typically found in the receiving email server's systems, and that they are rarely blocked on our end.
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