Emails are not sent from the Outlook inbox unless Send immediately is set

If Outlook is configured so that emails remain in the outbox after you click Send until the final sending is triggered, you may experience a problem if you open the respective email before sending.

In this case, the email will be put back into draft mode and must be put back into send mode by clicking Send before it can actually be sent.

This is not a specific behaviour of the Outlook Add-in, but a very general behaviour of Outlook and cannot be solved otherwise.