This contribs allow you to set few email transfer to external addresses without setting an internal user to handle it. It works almost as the pseudonyms. The only difference is that you can set multiple emails to send to for one alias, and you can sent ot an external email.
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Be careful before setting this, you will need to have a strong filtering, as you do not want to relay junk to another server as it could lower your server reputation. Also, it is better to handle the destination server too, as you might never know when and for what reason another ISP or mail service could consider what ou think to be regular mail to be spam.