multiple senders when sending emails

[Foni86]

Is your feature request related to a problem? Please describe.
Sending books to Kindle requires that all users register the same email as an authorized sender

Describe the solution you'd like
It would be great if each user could have individual senders if they want (the email they register with for example)

Describe alternatives you've considered
The user's email could simply be used as the sender automatically

Additional context
This software is fantastic and I have no idea how to program so I don't know if this is very complicated or not, but I thought putting it in might help more people. In any case, thanks for the work done.