|Email Plug-in (Reporting):||1.1.0-114|
|Email Plug-in (Encryption):||1.2.1-118|
You would need to enable recipient validation for internal users using a LDAP server which the ESA can query to.
The query can be enabled on the listener you use to accept emails.
Please refer to the below article for reference
if you don't have LDAP you can still use SMTP call-ahead.
Be aware that invalid recipient counts for maximum number of invalid recipients for DHAP attack prevention.
You can view the instructions on how to enable SMTP call-ahead on the ESA in the ASyncOS User Guide (link below).