[ZendTo] Support for custom mail headers

[Adam Thorn]

Would it be possible to add support for specifying custom mail headers 
to the mails sent by ZendTo? My specific use case is to be able to 
specify a header specific to Exchange Online which controls the sending 
of various out-of-office-type autoreplies..

https://learn.microsoft.com/en-us/openspecs/exchange_server_protocols/ms-oxcmail/ced68690-498a-4567-9d14-5c01f974d8b1

..on the basis that I know that header is useful for many of my recipients.

It looks like for PHPMailer one calls $mail->addCustomHeader() :

https://phpmailer.github.io/PHPMailer/classes/PHPMailer-PHPMailer-PHPMailer.html#method_addCustomHeader

whereas for php's mail() one just appends to the $header string that 
gets eventually passed to mail().

mail() expects a raw header line in the form 'Header-Name: 
Header-Value', whereas PHPMailer supports being called either with two 
arguments:

$mail->addCustomHeader('Header-Name', 'Header-Value');

or with just a single argument of the same form that could be given 
directly to mail():

$mail->addCustomHeader('Header-Name: Header-Value');

Thus, it might be simpler to go for the second form and to have an 
optional array one could specify in the preferences of the form..

$prefs = array(
   ...
  'extraMailHeaders' => array(
     'My-First-Header: Foo',
     'My-Second-Header: Bar',
   ),
   ...
);

which can then either be immediately passed to addCustomHeader() or 
imploded and added to $headers, as appropriate.

Regards,

Adam