It could be possible that you need to change server settings (server name, ports…) on one account in Outlook. What I am going to write is how I did in my Outlook 365 (it should work in Outlook 2016 as well).
I find two way to change server settings but i don’t know why, one works and the other doesn’t, in any case I will explain both.
Method 1: Modify Outlook settings from Control Panel (it works)
As you can image you have to open Control Panel and click on Mail (Microsoft Outlook 2016)
Click on “E-mail Accounts”
Select the account you need to modify and then click con “Change”
Now you can start changing server settings and if you need to change ports click on “other settings”
Here you can change ports
From now on just click OK and then NEXT to complete (Outlook will test connection to the server before the settings is applied).
Method 2: Change settings from Outlook (it doesn’t work on my systems)
Open Outlook and click on File then select the account you need to change and then select “Server Settings”
and in the end, change the setting you need
I hope It could help.