First, awesome plugin, a pleasure to use after cforms.
My contact form is very straightforward. Admin notification works perfectly.
The user notification only works with a specific email address in the "from email" field. To be more specific, this WP install is on an "add on" domain on my hosting account. I have created email addresses for both the primary domain and for this add-on domain.
When I put the email address from the primary domain into the "from email" field for the user notification, the email is sent correctly to the user and is received. When I switch the "from email" field to be the email address from the add-on domain, the user notification is not sent.
Note - I have the exact same email address from the add-on domain in the admin notification "send to email" and "from email" fields, and the admin notification always comes thru.
The only problem is with using any address from the add-on domain in the "from email" for user notifications.
As you are aware, most hosts do not support issues with WP plugins. However my hosting company (olm.net) did try setting up another email address for the add-on domain and had no trouble getting it to send. They did not (and I'm sure will not) try it in your plugin, however. And since they saw the add-on email address send properly they will no doubt tell me that it is the plugin.
Soooo, any ideas about the cause and/or resolution of this?