Sending mails and locating the buildings are too easy in Singapore as long as you have the postal code and your GPS (Global Positioning System). I am a forever fan of Google Maps. This application helped me a lot in traveling around the country. The postal codes usually indicate the building number so you'll never get lost especially when you are looking for an HDB unit. For example, if my postal code is 390047 meaning I'm at Block 47 and the 390 usually indicates the street or the area.
You can find helpful websites on how you can locate the postal codes. Try to check Geo Postcodes and Singapore Post.