sendmail: trusted-users or aliases?

For a project that I have been assigned to, I need to send emails to a business partner (business_partner.com) from one production server. However, my emails neither reach their destination nor bounce back to me.

Working with our business partner's IT support, the following error was discovered in their maillogs:

Further analysis by my IT support shows that emails are successfully sent out ("Message accepted for delivery"):

The app I coded is not using a public internet email address (e.g. me@hidden\_domain_name.com) to send these notifications. Instead, it uses an intranet email address (the server's where my code resides: user\_name@servername.hidden\_domain_name.com).

My IT support guys believe it is the cause of the problem. We created an alias but it made no change. Would adding my public internet email address to "trusted-users" file (we use sendmail) help?

How can I solve this?

Al.

Maybe mail direct for an IP with no reverse lookup, or with a HELO not matching reverse lookup, is rejected. Try sending to a more public email server of your domain first, and let it forward. He knows you and they know him.