Hi Rajesh,
SM : is the algorithm that preserve natively the uniqueness condition
SL : can preserve the uniqueness only if the number of values defined in you input file are superior or equal (prefer to have superior values) to the number of unique values you have to mask in your email column.
You can go with secure lookup, but this implies a maintenance to always ensure that the number of values defined in the algorithm are superior to the once in you email column.
Regards,
Mouhssine