Try 

s/(?<!_n)\@mail\.ab\.com/_n\@mail.ab.com/g;

[download]

The zero-width negative look-behind, (?<!_n) says only match if the @ sign is NOT preceded by the _n. Ie. if the address already has the '_n' it won't be modified.

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In reply to Re: Substitute problem by BrowserUk
in thread Substitute problem by Anonymous Monk

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