Outlook / Exchange displaying multiple copies of sent messages
When using Microsoft's Outlook.com and Exchange platforms in conjunction with Close.io you may see multiple copies of the same message displayed in your Sent Items folder.
It's important to note that messages are not sent to recipients multiple times, they only appear in your Sent Items folder several times.
You can prevent multiple copies from appearing in your Sent Items folder by disabling email tracking (Settings -> Email Settings -> Disable Email Tracking).
This problem is caused by the inability to IMAP search for RFC822 Message-ID. It is also not possible to IMAP search for any custom headers, for instance the X-CLOSEIO-EMAIL-ID, written to each message. Outlook.com also rewrites all Message-IDs created by the client when sending the message.