How should the service for converting the email body from base64 work?

Question

Programming

Hello.
In my opinion, there is no point in going into details, straight to the point.
A letter with base64 content falls into the mail client, I want to whip up a service to convert it into the encoding I need. (no web resources, only mail!)
Logic:
1. Received an email in base64.
2. Clicked forward to: [email protected]
3. Received a letter in the required conversion.
Any thoughts?
If it is local, then only windows, ideally without their own servers at all, but on a hosting.
PS Chukchi is not a writer.
UPD Not a writer of fiction, novels, prose, poetry, etc.