Prefer to use Microsoft Outlook for Mac or Mozilla Thunderbird for your email needs? No problem. MSG Viewer for Outlook works with all of your favorite email clients.
Just choose your email client or address book from the list under Preferences (Launch MSG Viewer for Outlook, and choose Preferences from the menu bar), and you'll be up and running in no time.
Gmail, Hotmail, and other Web-based email clients are not available options, see this FAQ for additional details. If you're curious why MSG Viewer for Outlook requires an external email client, please see this FAQ article.
Outlook MSG Viewer v1 and v2: legacy instructions are available here.