I received an "Undelivered Mail Returned to Sender" email, which is quite suspicious.
The message is displayed in Thunderbird like this:
From: someRandomStuff email@example.com
Date: 24.08.20, 21:05
To: a list of very random looking email addresses, probably machine generated
Is this simply a case of using some email address in the reply-to field, or is there something worse going on?