I'm using the same configuration on over 100 sites, but with a select set of email addresses, I get this error message when I try to make them a recipient:

~

This message was created automatically by mail delivery software.

A message that you sent could not be delivered to one or more of its recipients. This is a permanent error. The following address(es) failed:

< recipient's email >
No Such User Here

------ This is a copy of the message, including all the headers. ------

< rest of the message >

~

Thing is, I know the user exists, as I can email them manually (such as from Gmail) without issue. If I test the form I get this message auto-sent to the FIXED_SENDER address, not as an error message, which go to another address. I have my recipient emails stored in an INI file, so my TARGET_EMAIL array is blank, and I'm not sure what other setting to mention here as being the possible culprit. Here is the base form (it's shown in an iframe), and the session variables in the url are required as we have some session variables transferring across the iframe. Here's what I know:

The addresses that have this problem have nothing in common, not even their hosting providers. However, none of these addresses are hosted on our own servers, so I can't personally check their settings.

I know the session variables are working correctly or the form would have never gotten the correct email address from my INI list.

Of the 100+ email addresses we have as recipients to forms, this problem affects <5% of them.

And that's it, I think. Let me know if there are any other settings you need to know to diagnose the problem. I can't seem to find this problem elsewhere on this forum, so it's either very unusual or very easily fixed. Thanks in advance for any help.