Very nice.

I would make a couple of suggestions though. To avoid the loop looking at each individual character, use regular expressions to look for invalid characters:

IF (

@vParameter_Length = 0

OR CHARINDEX ('@', @vParameter) < 2

OR CHARINDEX ('.', @vParameter) < 2

OR CHARINDEX ('.@', @vParameter) <> 0

OR CHARINDEX ('..', @vParameter) <> 0

OR CHARINDEX ('.', REVERSE (@vParameter)) < 3

OR CHARINDEX ('.', @vParameter, CHARINDEX ('@',@vParameter)) = 0

OR RIGHT (@vParameter,1) = '@'

OR @vParameter LIKE '%@%@%'

OR @vParameter LIKE '%[!-*]%'

OR @vParameter LIKE '%,%'

OR @vParameter LIKE '%/%'

OR @vParameter LIKE '%[:-?]%'

OR @vParameter LIKE '%[[-^]%'

OR @vParameter LIKE '%`%'

OR @vParameter LIKE '%[{-~]%'

)

Then if you are going to do it on an entire table, you can avoid the function altogether which would probably bring your SQL Server to its knees to have a function in the where clause of a select against a million row table. This query ran against an 800,000+ row table in about 4 seconds.