SQL Constraints and Check Help

I need assistance with writing sql constraints in a table already has the rows and columns populated.

The constraints are:

a email contains the @ symbol for all entries of a column in a table.

the entry in one column of a table is greater than or equal to the entry of another column in the same table

A column only has the ability for three options to be entered, such as 'pending' 'approved' or denied

When three columns are unique to a table Nulls are only allowed in the column if the column above is at 'pending' status.