How does Mail que/drop work? Advance.

How does a mailing program such as exim know if your sending mail to existing email adddress but is down so puts it on a que or doesnt exist and returns the such no email to sender??

There must be a working way, I got a single server and i dont want miss mail if i go down for a reboot or server gos down for hardware upgrade.. Trying to find out how mail programs reconised such address exist or dont exist on the internet to put it on que retry.

 

 

 

 

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