When viewing the logs, a large number of warnings of the following content were found:
Invalid sender. "@" is missing. BATV tagging will be skipped
What is this error and how to get rid of it?
Following warning refers to Bounce Attack Prevention technology to eliminate non-deliverable receipt (NDR) messages that are a result of redirection techniques commonly used by spammers.
It can be avoided safely as SMG shows the following when an email does not contain a "Bounce attack signature" to validate the email hence is the reason for the warning.
More details about Bounce Attack Prevention can be found in the Administration guide on page 132.