If you need to split something into 3 pieces, such as an email address and you don't know the length of the middle part, try this (I just ran this on sqlserver 2012 so I know it works):
SELECT top 2000
emailaddr_ as email,
SUBSTRING(emailaddr_, 1,CHARINDEX('@',emailaddr_) -1) as username,
SUBSTRING(emailaddr_, CHARINDEX('@',emailaddr_)+1, (LEN(emailaddr_) - charindex('@',emailaddr_) - charindex('.',reverse(emailaddr_)) )) domain
FROM
emailTable
WHERE
charindex('@',emailaddr_)>0
AND
charindex('.',emailaddr_)>0;
GO
Hope this helps.