[Feature Request] Add custom headers

[ncou]

Hi,

Is it possible to add a new feature to add custom headers ?

I think to add a custom header to check after the email send, the percent off "mail bounce" using this header to check via imap the returned mails.

[swt83]

Can you show me some examples?

[ncou]

Here about the example :
https://dyn.com/blog/what-are-x-headers-email-delivery-101-why-do-they-matter/

And here for the spec :
http://www.faqs.org/rfcs/rfc822.html#b

PHPMailer has a method "addCustomHeaders()" here https://github.com/PHPMailer/PHPMailer/blob/10341cc3106916a218c4fc3394f674adf127d625/src/PHPMailer.php#L3638

Looks like the custom header should start with "X-".

[swt83]

I don't have any objection, if you want to add this ability and submit a pull request.