Email submission service allows any From
While it might be a nice gesture to allow users of online services in general to chose the way to present themselves, we don't want to provide this on our email submission service. The way spam is fought today is not limited to what words are being sent, but also what server is sending them. And while we do authenticate submitters of email messages, we don't enforce what From: headers they put in.
We could use the postfix smtpd_sender_login_maps
config option for mapping From addresses to user id's.