I'm not exactly sure how you're choosing these items, but its sounds like this might be close to what you're looking for. When you're choosing the item, you could do something along the lines of:
If(
EndsWith(
YourSelectedItem,
"gmail.com
),
Notify(
"This email contains gmail.com"
)
);
Where YourSelectedItem is the email of the item you just chose.
This would briefly display a message to the user informing them of this.
If you wanted a lasting popup that the user has to interact with, replace the Notify() with Set(Variable, true). Then, make a container, set the visibility to be Variable you just created, and customize it as you'd like. Once the user has done what they have to, set Variable back to false and it will become invisible again.