This sounds more like an issue with your email account setup in Outlook, the GoToMeeting plug-in simply creates a meeting invitation and inserts the GoToMeeting details. This would not affect your ability to send the email itself. If I remember correctly when an email is stuck sending in the Outbox you can usually click on it to see if there is an error message explaining what is wrong.
@P-Dario Have you already tried reinstalling Outlook from scratch?
Does similar behavior present itself when you are copying other text into an Outlook appointment?
@P-Dario Do you have any error messages or screen shots that may help us understand what Outlook is doing there? If not, could you reach out to Customer Support in order to demonstrate the failure?
@P-Dario If this is an ongoing issue, I would consider the following steps: