When email is rejected (or summarisation rejected) it will appear on the MTA report for your domain(s). There are a relatively small numbers of causes because they are aggregated and can be divided into two classes, pre-envelope is before we have the complete envelope and is very basic validations, and post-envelope which is after we receive the email content. Here's a summary of the common issues:

Pre-Envelope

Host Not Found or Client Host Rejected: Some errors indicate that the host (server) could not be found or rDNS is setup incorrectly or not at all, or is on a banlist, is flagged as dangerous or has really poor reputation, such as:

• cannot find your hostname 208.113.158.108.

System Unavailability: There are very rare instances where services are unavailable or overloaded especially if they are misconfigured, and this can lead to pre-envelope rejections.  

Post-Envelope

Undeliverable Addresses: Indicates that the sender or recipient address is invalid or doesn't exist (e.g., "undeliverable address", "bounce-mktg 14081855.43").

Unverified Addresses: Indicates that the sender is unverifiable due to being sent from unknown or untrusted sources, or a source with a bad reputation or that fails SPF or DMARC checks. 

Blacklisted IPs/Domains: Indicates that the sender or their sending server(s) are listed on various blacklists, such as Spamhaus XBL and Barracuda Central. We use over 20 blacklists to aggregate reputation before making automated decisions. For example:

1. 77.32.166.160 is blocked by Barracudacentral.org.
2. 35.201.213.1 is listed on Spamhaus XBL.

Access Denied: Indicates that the sender or their server was rejected due to specific access policies, such as:

• AmyGould-aldurld1hhutthcli1y cmail20.com access denied.
• ilhaxct restore-eu.my access denied.

Spam Filtering Issues: There is a comprehensive SPAM classification and detection system that ranks email based on its 'risk' and where that risk is deemed unacceptable, a rejection occurs. 

Please refer the our our rejections FAQ for the NDR content and our other articles on email delivery and protection. 

If you need further assistance with specific parts of this issue, feel free to raise a ticket at the HelpDesk.