Here are some steps that may help to resolve this issue:

1. Check if the email has been sent to a large mailing list
2. Check to see if the email contains any images or attachments that might have caused the blocking
3. Check if the email is being sent to an invalid or non-existent address
4. Check for any suspicious content in the body of the email that could trigger spam filters
5. Check the header of the email to ensure it does not contain any suspicious links
6. Check for any blocked IP addresses that may be responsible for the blocking.