Perhaps extend nostr:npub1sy70twa0vadtk8hjs6wt2hmfszduj04tw78ccs3ktmr9u99mfmqsj62srx 's example, by referencing attachments, in the context of email / the SMTP protocol...

Sending attachments is something the SMTP protocol supports, but emails are not obliged to contain attachments. As a matter of fact, the majority of emails probably don't include attachments...

Similarly, Nostr supports Bitcoin, as a feature, at the protocol level, but sending Bitcoin is entirely optional, and in the majority of use cases, it's probably not relevant...