Ability to send mail to an (API) file instead of mail() or SMTP
The ability to send mail to an file which allows us to send e-mail using a mail provider API instead of using the default mail() or SMTP function.
Main reason to do this is when WHMCS is used on shared hosting environments remote SMTP servers are likely to be blocked and the PHP mail() function is most likely not configured as it should be to ensure e-mail deliverability. When using a mail provider API the mail can be send to the mail provider without the need of mail() or SMTP functionality and it adds to overall mail security and improved mail deliverability.
This function/option should basically just send all e-mail data including attachments to a PHP file where this details can be used to send out the mail through the mail provider API.