Is this a problem that has only recently occurred? Please make sure you have upgraded your Outlook 2016 to the latest version.
Does this issue happen to each email you sent? Or only happen to some special emails?
In order to confirm if the issue is related to your Outlook client, please try to log in to your web mail and send emails to see if this issue colud be reproduced. You can also try to add other email accounts to the Outlook client and see if the has any difference.
As I know, the send/receive file (.srs) of your account may have effects on this issue. Please try to delete the .srs file and restart outlook client to see if it works.
File location: drive:\Users\<username>\AppData\Roaming\Microsoft\Outlook
There could be some add-ins in your Outlook client that cause this issue, so it is suggested to start your Outlook in safe mode (Press Win + R, type “outlook /safe”, press Enter.) and see if the issue continues.