Have Outlook of 2016 or older.

1. Install the latest web tools:

2. Go to SuperOffice CRM > create an appointment > include one internal participant (internal employee of your company) and one external participant (external user from a different company).

3. Send the invitation.

Observed that: internal users receive invitations correctly based on the template. However, the external recipients - will receive a such invitation: 

NOTE: When using older version of Web tools, for example, Web tools 12.2 - it works as expected.

Expected that: the invitation would work as expected when using the latest web tools with an older version of Outlook.