To resolve this issue, do the following in the Database Mail Attribute:

1. Log in to the IMSVA shell via the Command Line Interface.
    

   When logging in to the IMSVA shell or Command Line Prompt, make sure that you have root privilege.
2. Run the following command:

   # /opt/trend/imss/PostgreSQL/bin/psql imss sa

   Or

   # cd /opt/trend/imss/PostgreSQL/bin/
   # ./psql imss sa

3. On the "imss=#" prompt, type:

   update tb_global_setting set value='proxyAddresses' where name='mail_attr';

4. To quit, type "\q".
5. Log on to the IMSVA web console. Disable recipient checking and then save the settings.
6. Go back to the IMSVA web console to enable recipient checking again and save the settings.
7. Check and confirm that the issue has been resolved.

If the above steps did not resolve the issue, then do the following:

1. Export the Address Group.
2. Contact Trend Micro Technical Support.

If you are using InterScan Messaging Security Suite (IMSS) for Windows, refer to this article:
IMSS 7.0 for Windows users get "SMTP Error: 550 recipient was not found in LDAP server".