Outlook Custom form Issue: Receive "Operation failed" error message

When you send an item that is based on a custom form, the form is not sent and you receive the following error message: "Operation Failed" 

This issue will happen if, a Recipient field on the custom form is configured with an initial value and with the Calculate this formula automatically option for that value.

Please view this useful MSDN KB Article. It has the information about the issue, its cause, workaround solution for this issue.