I now realize that OP posted a select few, safe for work images here on Lemmy.
The joker one im talking about is this one:
(In case I didn’t get the image embedding right, here is the image as a regular link: https://cdn.masto.host/fedggtmeu/media_attachments/files/111/566/629/459/550/195/original/5505dd45f0c1d22b.jpg)
I think it’s fair to prevent users from causing mail sent to your internal systems. It probably won’t cause any issues getting mail to the machine inbox for (no domain name), but it reasonably makes security uneasy.