When the RAM levels are high, you may quit the app using Cmd + Q when the app window open. Then simply reopen the app from fresh and RAM usage will be lower.
The RAM usage you see in Activity Monitor for MailTab is mostly 'inactive' RAM. This is RAM that has previously been used by the app and has been stored by Mac OS X for quick access should the app need to access that same data again (such as cached images from emails).
However, because it is 'inactive' memory, it means any other app can use that memory if required. So please don't email us complaining about MailTab using lots of RAM. It is not. WebKit (the framework built by Apple) is caching images and other data loaded within the website, and will release this data if another application you are running requires more RAM.