Mailplane 3 is a major upgrade and requires a new license. Our previous customers are eligible for a 20% upgrade discount and all customers who bought it on or after October 1 2012 will get the upgrade for free.
You're very welcome to download everything we've ever made. But be warned that we don't actively sell and develop Mailplane 2.5 anymore.
Mailplane 2.5 updates are still available through the Mac AppStore. Open AppStore and switch to the Purchases section.