Postage is less reliable and much more expensive than it once was and can take quite a while. Postcards can reach Europe in four or five days, but they might also take two months. Post boxes are rare except outside post offices. Poste restante is unreliable. If you are sending mail to an Iranian address that's complicated or remote, try to get the address in Farsi.
Sending a parcel from Iran can involve much form shuffling, but your package will usually arrive. Take your passport and unwrapped goods to the parcel post counter at the main post office in a provincial capital before 2 pm. They will be checked, packaged and signed for in triplicate. There are three parcel services - pishtaz (express), havayi (airmail) and surface. Rates can vary, but a 5kg parcel to anywhere by surface mail will cost less than US$100; airmail is more expensive. The customs officer on duty generally has discretion over what can be posted abroad, so be nice.