I suggest you ...

provide an event to let Outlook Add-in respond to a change in the message being written (for example message body, To, Cc and Bcc lists)

As referred to the post:
https://social.msdn.microsoft.com/Forums/office/en-US/720a1bd6-abb9-4f75-863f-1fc33256a629/is-there-an-event-for-when-recipients-change-in-outlook-2013?forum=appsforoffice

there is no event JavaScript API an add-in for Outlook could use to react to a change in email message being written (compose mode).
For example if the user adds further To, Cc or Bcc Addresses, it would be advantageous if the add-in could react to it and display relevant data.

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    Zbynek shared this idea  ·   ·  Flag idea as inappropriate…  ·  Admin →

    2 comments

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      • Rick Goud commented  ·   ·  Flag as inappropriate

        Is there an update on this issue? Would be great if there is no manual trigger needed to activate an add-in in compose mode just like in read mode. Why is the priority low given te relatively high number of votes? Is there an overview what then has higher priority?

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