Hmmm, this is usually something that's done server-side, in my experience. For instance, GoDaddy's email service allows you to do this when you setup a new email account. I am just not sure of any open-source (I am assuming that's what you're looking for) email program that most hosts use that could do this.
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thanks! but may i know which squirell mail version are you using? coz i see it says that the 'compatibility' plugin also must be installed but it only come with the patch for version up to 1.4.6 while the current avaliable version for download is only 1.4.9