Outlook Calendar event invitation mail gets deleted upon event acceptance through Calendar app on iOS

Currently the Outlook Calendar event invitation generated by outlook gets sent to the invitees through email. That invitation generates a calendar event on the recipient account.
If the recipient now accepts the invitation through the calendar app on their mobile phone, the acceptance is sent as a mail and the incoming mail invitation also gets deleted automatically.
This auto deletion of invitation mail happens only for invitations generated through Outlook and not from any other meeting invitation generation client like Yahoo even if used on the same native Calendar app on iOS.

This deletion of mail automatically is erratic and should not happen.

What should I do?