I perfectly understand why a contact form would reject a mail with the domain "example.com". This is obviously not a valid email domain (and may be the default domain used as greyed example in the contact form).
example.com was only used to take an example screenshot.
[1] https://adamjones.me/blog/dont-use-contact-forms/#:~:text=ma...
[2] https://montreal.ca/sujets/politique-de-confidentialite#:~:t...