My main requirement is verification of the sender. One way to do that could be to send only a link to the message and have the receiver request the data from senders server. Then receiver could cut it off if the file is too big. (Maybe notify end user of the undelivered mail and they could retry if they're OK with it). Not sure how long a pending message should stay on the senders server.
Another idea is to have the payload be a zip file. The end user could then have apps to process different content types. Only the "email" type would get processed by a traditional email client. Attachments would just go in a folder under the email message.
Just thinking out loud. An authenticated asynchronous method of sending "stuff" including "email" messages.
It's all internet-centric. You can't download from the sender if the sender is an onion service and you're getting the email through an onion-to-clearnet relay, even if the sender address is correct.
And MIME is already like a zip file but different.
Another idea is to have the payload be a zip file. The end user could then have apps to process different content types. Only the "email" type would get processed by a traditional email client. Attachments would just go in a folder under the email message.
Just thinking out loud. An authenticated asynchronous method of sending "stuff" including "email" messages.