On app start
Set(varUser, Office365Users.MyProfileV2().mail);
ClearCollect(Agreements, Filter(DataSource, varUser in datasource.emailcolumn)
Then in your 'pop up' warning label, set the visible property to
If(
CountRows(Agreements) >0, true, false
)
This will collect the items in your list only for the logged in user. Then CountRows will count the rows in the collection. If there are more than 0 rows, the warning will be displayed