Office 2010 Compatibility Error: The Mimecast Exchange Fix

Fixing “Default Account Is Not an Exchange Account” With Office and Mimecast

If you’re running Office 2010 or Office 2013 (including Project or Visio) alongside Mimecast for Outlook and hit an error stating the default account is not an Exchange account, this is a known compatibility issue between the older Office version detection and the Mimecast add-in, rather than a problem with your actual Exchange or Mimecast configuration. Because the specific registry fix for this depends on your exact Office build and Mimecast for Outlook version, the most reliable route is to check Mimecast’s own support knowledge base for the current article covering this error — Mimecast maintains and updates the exact registry key and value for each supported Office/Mimecast combination, and using an outdated fix for the wrong version combination can cause more problems than it solves. Before diving into the registry, it’s also worth ruling out the more common causes first: confirm the Outlook profile’s default account genuinely is a full Exchange (not IMAP/POP) mailbox, that Outlook itself opens and connects normally without the Mimecast add-in enabled, and that you’re running a currently-supported build of both Office and the Mimecast for Outlook client — Mimecast periodically drops support for older Office builds as Microsoft itself retires them.

Major Update: Mimecast No Longer Supports Office 2010 or 2013 At All (Updated for 2026)

This is worth flagging clearly: Mimecast’s own current Mimecast for Outlook prerequisites documentation lists Outlook 2016, 2019, 2021, 2024, and Microsoft 365 Apps for Enterprise as the supported client versions — Office/Outlook 2010 and 2013 aren’t listed anywhere. That’s consistent with both products being long past Microsoft’s own support window (Office 2010 since October 2020, Office 2013 since April 2023). In practice, this means the specific compatibility error covered above is now a legacy scenario rather than something Mimecast maintains an active fix for on a supported configuration. If you’re still running Office 2010 or 2013 with Mimecast today, the real fix isn’t a registry tweak — it’s planning an upgrade to a currently-supported Office/Microsoft 365 Apps release, both because Mimecast won’t officially support the older client and because those Office versions themselves haven’t received a security patch in years.

Resources

• exchange