A user can't open a calendar from a sharing invitation in Outlook on the web because the Accept button is missing. Additionally, an .eml file is attached to the sharing invitation.
This issue occurs if a disclaimer transport rule is applied to calendaring messages, and Wrap is specified as the fallback action.
To resolve this issue, do one of the following:
Change the fallback action of the disclaimer transport rule from Wrap to Ignore.
Change the disclaimer transport rule to skip calendaring sharing invitations. For example, add an exception that specifies the following conditions:
Except if: The subject or body > subject or body includes any of these words > has invited you to view his or her Microsoft Exchange Calendar or Sharing invitation or Sharing request or I'd like to share my calendar with you or You're invited to share this calendar
For more information about how to set up disclaimers, see Organization-wide disclaimers, signatures, footers, or headers.