I disagree with earlier posters. When email is auto-forwarded, the original sender will receive a delivery error message if the destination server refuses the message. I just tested this as follows:
- I set up an auto-forwarding rule at a Fastmail account. This could have been at any email service which supports automatic forwarding.
- The forwarding rule targeted a non-existing gmail address.
- Now when I send test messages to the test account (from any email system — I tried using my Yahoo account), the forwarding to gmail fails and the delivery failure message is sent to the original sender. This message comes from the system which is attempting the transfer which fails (in this example, the failure message is sent by the Fastmail system to my Yahoo account).
This is the standard way that email operates. If you look at the full headers of received messages, you will see that a typical email message is forwarded through many different servers. If any of those transfers fails, the failure message is sent to the email address in the “FROM” header.
So all that is happening is that someone has set their email account up to auto-forward to a different address, and the final destination address is no longer accepting messages. There might even be several levels of redirection, just as the typical email delivery system uses. It makes no difference that the redirection is set up by an email end user — any failure to transfer the message to the destination results in a service message to the sender in the FROM header (added by the original email sender).
It would be good to inform that person that their forwarding is failing. They might not know this is happening.
Bill