Currently there is no way to enforce rules on data entry.
Proposed solution:
a) Action to dynamically display pop-up with message.
b) Ability to check the existence of a value in a column
- Required to avoid running an automation as well as to display a popup
c) Ability to check existing of a specific value in a column (as well as a formula column) - Formula column would allow true false verification of information entered in multiple columns such as email address and verification email address.