The Inreach email address is not a true email address, you can not just send an email from a computer to this email, it appears to only be good for Inreach to Inreach communication. Not sure if it would work with the spot but it doesn't work like a normal email address. The Inreach email is not a proper way to contact an Inreach, even states this in the manual. Only way to contact an Inreach is via Mapshare, respond to a text message, through Garmen Explore or Inreach to Inreach, all 4 require prior planning.
Like you stated the phone number that appears with a text message doesn't work like a normal phone number either, try taking the number sent to a phone and type it into another phone and send a text. You will get back an error message.