I have a form that works fine when the admin (not the from) address is set to user@domain.com but fails if the admin address is changed to user+x@domain.com even though that address is valid and works from mail clients, etc. What could be causing this?

It's not a problem with my provider– the mail works fine, including from my own PHP scripts and from your script… there's something specific to YOUR plugin that is breaking this by not sending email if the email has a perfectly valid construction with a + in it. IF the only thing I change is the email address in the cforms settings and then it stops working while working everywhere else, how is that my provider's fault?

Did you even try this to confirm the problem? I've duplicated the problem on three different servers. Coincidence?

OK, I was able to verify that this is a cforms problem. You can see it for yourself by putting an offending address (x+y@foo.com) along with the (x@foo.com) and viewing the headers in the email that will arrive at the second address.

I also dumped a copy of the mail to a text file.

What's happening is that cforms is stripping the x+ part from the address, so mail to:

john+foo@bar.com or John Boy <john+foo@bar.com> 

is being stripped so that the resulting header becomes:

foo@bar.com or John Boy <foo@bar.com>

which is undeliverable of course.

I'm unsure how I could literally be more precise than "user" and "user+x" ! The +form of an email address is part of the RFC specification.

But I appreciate the patch in advance.