For HDB flats, mails arrives in mailboxes that looks like this one.

This reminds me of the postcards that I received from a few Daily Photo bloggers around the world. Have received postcards sent from: Edwin - Kuala Lumpur DP, Jing - Shanghai DP, Jazzy - Tuzla DP, Gerald - Hyde DP, Zannnie/Zsolt - Budapest DP.
You want to see some unique Singapore mailboxes for sending mails? Watch out for Theme Day on 1 April.