Troubleshooting “Mailbox unavailable. The server response was: Access denied - Invalid HELO name” when sending email with SmtpClient

Question

I have been trying to send an email by C#. I have Googled for various examples and have taken bits and pieces from each and from the standard code which everyone would most

Answer 1

If you have a webmail client available and you see a cPanel logo it could be a setting there as well.

We got the exception and asked our hosting company to go into:

"Root WHM > Service Configuration > Exim Configuration Manager > Basic Editor > ACL Options"

and set the Require RFC-compliant HELO setting to Off.

This worked for us after fixing the next error:

SMTP AUTH is required for message submission on port 587

Source:

https://serverfault.com/a/912351/293367