s/@/\/@/g;
[download]

But the real problem lies in how you declare $string, that is, the @ sign must be escaped if you use double quotes:

$string = "foo\@bar.com";

# or better yet:
$string = 'foo@bar.com';
[download]

UPDATE:
_NOW_ i see the comment that says populated by DB query, sheesh!

Jeff

R-R-R--R-R-R--R-R-R--R-R-R--R-R-R--
L-L--L-L--L-L--L-L--L-L--L-L--L-L--