Outlook error: 'error preparing to send sharing message'

When users try to send a sharing message after sharing the calendar with another user and they receive a message that there was an error preparing to send sharing message, you need to remove the delegates rule using MFCMapi. You'll need to use online mode with MFCMapi. (Disabling Cached mode should suffice.)

1. Log in into the profile using Session, Logon
2. Right-click on your account's message store and choose Open Store or Double-click on the account's message store to open it.
   
3. Expand Root Container
4. Expand Top of the information store
5. Right-click on the Inbox folder and click Display Rules Table to see all the rules configured in the mailbox.

   

6. Scroll right to see the PR_RULE_PROVIDER column or scroll down in the lower section.
   
7. Right-click on the Delegate Rule where "PR_RULE_PROVIDER" column property shows "Schedule+EMS Interface" and choose Delete
   

This removes all delegates and the user will need to re-add delegates.

This method can also be used to delete orphaned delegates from a mailbox.