Duplicate To header
Status: Beta
Brought to you by:
evyncke
The implementation example in dkim-test.php creates a duplicate To header.
The To header is inserted in the $headers variable, but is also automatically added by PHP's mail() function, so when the mail goes out, the To header is listed twice.
Some ISPs like Gmail ignore it. Some ISPs like Yahoo deliver it twice (and may fail DKIM validation)