Mailsuite is no longer available in the Opera add-on store.
You can still use Mailsuite with your Opera browser. Here's how:
1) Open Opera and go to https://addons.opera.com/en/extensions/details/install-chrome-extensions/.
2) Install the Install Chrome Extensions extension.
3) Once you've done that, go to https://chrome.google.com/webstore/category/extensions and search for Mailsuite.
4) Click on the ‘Add to Opera’ button.
5) Click on the ‘Install’ button in the Opera Extensions Manager.