With the exception of Google and Plug-in (e.g. Rapportive, Boomerang, etc.) Mailplane opens links externally in your default browser. You can override the default by either holding the
Command key (= force internally) or the
Command and Shift key (= force externally) while clicking the link.
Here are the steps:
Click a link (e.g. Evernote Market link)
It will open the page in your default browser (Chrome, Safari or Firefox).
Command-Shift(externally) if you want to override the default behavior.