WordPress not sending mail from add-on domain

When a new WordPress user registers, an auto-generated password is sent to them. But if WP is installed in an add-on domain, the welcome message is never sent. A simple PHP mail test from the add-on domain works fine.

There's a thread on WP not sending welcome messages here:
http://wordpress.org/support/topic/24981

But I'm not having a lot of luck getting the fixes to work, and am also reluctant to hack the source code (don't want fixes to break on next WP upgrade).

Anyone have experience with this? Theories?

Thanks,
Scot

 

Here's a more distilled description of the problem, with additional information:

WP installed in cPanel add-on domains do not send welcome messages to new WP users.

Watching the exim_reject_log shows:

F=<> rejected after DATA: there is no valid sender in any header line

...

Envelope-from: <>

...

F From:

phpinfo(); confirms that the sendmail_from property has "no value."

Adding this to .htaccess :

php_value sendmail_from user@domain.com

causes phpinfo() to show a valid value, but attempting to register a new user still shows exim refusing to send the message because there is "no valid sender."

A simple PHP mail test script in the same dir works fine.

Anyone else seeing this, or have ideas?