I recently had an idea for a similar service that uses a storage account of your choosing as a backend. The plan was to market it as a “vault” for your emails and attachments.

Basically, you get an email address, and any email you send to it is streamed to a storage account of your choosing, with optional minimal processing (e.g., convert your emails into Markdown prior to storage).

I spent a while setting up the mail processing backend using Postfix + a mail filter written in Rust.

I ultimately ended up scrapping the project because:

A) It didn’t seem like there was a large enough market of paying customers.

B) There already are a few services that do something like this, but with a focus on attachments.