So a programming ERROR “is expected”.
Glad you folks aren’t building airplanes.
Let’s try this again. It is a BUG and it should be fixed.
There are dozens of valid top-level domains: https://en.wikipedia.org/wiki/List_of_Internet_top-level_domains
In fact, all they have to do is remove the top-level domain check. The customer knows what they need and
A successful test message will validate or not the email configuration data.