Uhm... the line you gave will work. I expect you are inadvertently running it on a single page more than once. You could modify the regex to prepend the "_n" only if those two chars aren't already "_n" by using a negative look-behind assertion:

s/(?<!_n)\@mail.ab.com/_n\@mail.ab.com/gi;
[download]

-sauoq
"My two cents aren't worth a dime.";