We are building the first open-source email alias and identity provider service, called SimpleLogin
It works in a similar way to other email alias solution (spamex, mailcare, e4ward, etc) with Postfix as MTA and Python script to handle the email forwarding AND backwarding (we don't know how to call it yet, any idea here?): basically an alias can both receive and send emails.
The code source for both the server and clients (libraries, browser extension) is available on our Github at https://github.com/simple-login/, feel free to check it out! We spent quite some time on the self-hosting instructions, if you have your own server, would really appreciate if you can try it!
Please let us know if you have any questions/feedbacks/critics.