To provide you proper solution, could you please share the screenshot of the exact error message you are receiving? and if you can't specify the error then try the following work-down:
Make sure that all the related parameter to the outgoing server: Port number and Encryption type are correctly mentioned. If they are incorrect then use the correct server settings of the account which is configured in your Outlook 2016. Now try to send the email.
If you till face the same issue then it may be due to your corrupt Outlook profile. To resolve it, create a new profile but before that don't forget to delete the old profile by which the emails are not being sent.
After trying the above method, if the problem still persist then check the sync between Outlook and server. For more info, read this informative article: <removed>, which explains all the related issues to send/receive error in Outlook along with all possible solution.