Email on Standard is different because Email cannot be sent through a relay like other requests are. Email uses the SMTP protocol and will be received by an SMTP server. On the Standard Public Relay, you receive an @standardcomputers.net email and these emails go to the SMTP server online. This SMTP server is custom built to forward these emails to your device queue in the Public Relay as a standard request.
Once emails make it to your device, they are saved in the email standard. Attachments are saved in an attachments folder outside of the Files sandbox and are only accesible via search or directly accessing the email standard.
Custom Relay Email
At this time, if you are using a Standard system that is not on a Standard Device, you can not email your Standard device. You must use an SDK or API to get emails to sync to the email standard on your Standard system.