It is not hard to create email addresses where Checksums overlap.
if checksum('email@example.com') = checksum('firstname.lastname@example.org')
print 'Checksum function is weak'
If you really want to do something like this, I'd recommend hashbytes:
if convert(bigint,hashbytes('md5','email@example.com')) <> convert(bigint,hashbytes('md5','firstname.lastname@example.org'))
print 'Hashbytes creates a better number space'
But really, I think the overhead that is introduced in this process is counter productive. Why not just index the column? SQL server will do the hashing for you in a way that matches your collation. SQL automatically computes, stores & maintains the hashes for you in a b-tree when you make an index on a varchar column.