I keep getting emails with the Subject header and the following message
Well, I am not if that is caused by ElkArte but do wait for others to response.
It means exactly what it says ...
ElkArte sent the email to the supplied email address. The server at that address, in your example gmail.com, replied that no username abcd exists so gmail can't deliver it.
https://www.youtube.com/watch?v=PU5xxh5UX4U
Oh ok, then the spammers are trying to sign up and the confirmation mail is not sent to them as their email ids are fake.
Elvis.. Lol 😂