Use Email Track and Trace or check the email headers to ensure that the email was scanned by Symantec.cloud.
Forward a copy of the email as attachment to email@example.com.
If you do not want to receive email from the sender in the future, add the email address to the appropriate blocked sender list.
If you believe the sender has already been added to the blocked sender list, confirm that you added the envelope sender address to the list. You can confirm this by looking at the sender as reported in Email Track & Trace.
Manually submit Threat-based false positives or false negatives