Add Mail gateway

[x-way]

Thanks for providing this nice ftpserver framework, I used it to build a basic FTP-to-Mail gateway which might be useful for others as well.

The story behind it is that we have a Brother printer-scanner-copier multifunctional device which is IP enabled.
It comes with a builtin Scan-to-FTP functionality but no Scan-to-Mail functionality.
With this basic FTP-to-Mail gateway in place there is now a one-button-press Scan-to-Mail functionality available 🙂

Additionally since everything is done inline, when the mail sending fails for some reason, this failure is reported back via the FTP response to the scanner device and immediately shown on the display to the user 🎉

[fclairamb]

Hi @x-way,
I love the idea !!!

The whole idea behind the ftpserver & ftpserverlib is to connect things from different decades. I was more thinking about connecting it to cloud services but emails are definitely a relevant use case.

And yes, I think reporting the SMTP sending status back to the FTP client at the end of transfer seems like a very smart choice.

Thank you for your contribution and for taking the time to go through my evil linting setup.

What comes to mind when I see this is that I should add the ability to write to two different "fs" / backend at once. So that we can both send it to email and archive it on S3 or SFTP.