Some external domains reject connections when there is an SMTP email address in the SMTP HELO command. The MTAs reponse to an email address in the HELO command varies. Microsoft SMTP server rejects it immediately, while Postfix accepts it. However, there are MTAs with stricter polices because "@" is not a valid character for a hostname or FQDN.

The SMTP requires MTA server to specify its hostname or FQDN in the HELO command, so stricter MTAs can really block emails with "@" character.

To resolve the issue:

1. Login to IMSVA command line interface.
2. Go to /opt/trend/mss/postfix/etc/postfix.
3. Open the main.cf file.
4. Ensure that the parameter below has no at sign (@). The default value should be "$myhostname" to use the actual value of "myhostname" parameter.

   smtp_helo_name=<mail server name>

5. Save the changes.
6. Restart the Postfix by running the commands:

   postfix stop
   postfix start