The @deleteduser.com example is the easiest to explain
A user, Bob, has an account and has attached his email address. Later, Bob says he wants to delete the account. For legitimate reasons, the company doesn’t want to delete all associated records (think, user generated content or user to user interactions that shouldn’t simply disappear. Or maybe audited transactions.)
The company also doesn’t want to have those associated records pointing to nothing … maybe the schema doesn’t support that (eg required foreign key constraints) so it has to point to something
So the company changes Bob’s account details. They delete his profile picture, his phone number, etc. but Name and Email are required by the database. Their deletion process changes Bob’s name to deleted_user and they change his email to deleted@deleteduser.com thinking that’s a dead domain that’ll never be valid to receive emails.
Later, another process triggers an email. Someone sends Bob’s account a message, replies to a thread, etc. this process doesn’t know / care that Bob’s account was deleted so it fires the email to deleted@deleteduser.com
There are a lot of problems with the decision making here, and there are better ways to handle all of this. But these are all rational decisions in the moment that can cause an email to be sent to an “invalid” email address