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.
When you send a email, exim tries to deliver it, and if it cant deliver it successfully, it will place the email into the queue.
If it connects to the remote server its trying to send to and gets a return message like 'there is no user here', then it will drop it and send a return email to the sender (you). However if it cannot connect to the remote server it goes into queue.
Once it is there, exim will retry sending the email a certain number of times until it gives up after a long retry period or until you manually clear the email queue.