Mimecast can block an email delivery with the following error:
Error 501: Invalid Email Address.
This implies that either the email address does not exist, or, the structure of the email address contains characters that cannot be used within an email address.
If you make use of distribution list in Synatic, its important to check that you don’t have any spaces (" ") preceding the email address.
E.g. firstname.lastname@example.org; email@example.com vs firstname.lastname@example.org;email@example.com
Noting the spacing between the two email addresses. Removing this space should resolve this issue.