What does mail delivery subsystem mean in email systems?

[XavierMeau]

I received a message from mail delivery subsystem after sending an email. I’m not sure why mail delivery subsystem returned the email or what the mail delivery subsystem error means.

 

[robbinhook]

A Mail Delivery Subsystem is an automated feature in email services that sends messages about delivery issues. If your email fails, this system generates a bounce-back notification explaining the problem, such as invalid addresses or server errors. Platforms like Gmail or Microsoft Outlook use it to inform senders and help troubleshoot delivery failures quickly.