Update your SMTP port, SSL settings and SMTP Authentication in Outlook 2013-2019

HIS servers are using an SSL certificate issued to HIS: *.his.com.

Make sure Outlook uses the HIS server name ending in his.com instead of your domain. so it'll match the certificate name and will not trigger an error.
HIS server names can be zmailX.his.com for Zimbra-hosted email or psaXXX.his.com for Plesk-hosted email , where X is a number.

Open a Support ticket at https://support.his.com/contact to find out your HIS mail server name.

Here is how to update your SMTP port, SSL settings and SMTP Authentication in:

Outlook 2016-2019
In the new simplified Account Setting dialog in Outlook, the password field and server settings are no longer accessible through the File, Account Settings dialog.

Here is how to update those settings:

1. Start Outlook.
2. On the File menu, click Account Settings> Manage Profiles to open the Mail Setup > Email Accounts.
3. Double-click on your email account from the list then to open the old Change Account dialog.
4. On the Change E-mail Settings window, click More Settings.
5. Click the Outgoing Server tab.
6. Make sure the server name ends with "his.com".
7. Check the My outgoing server (SMTP) requires authentication option.
8. Click the Advanced tab and change the Outgoing server (SMTP) port.
   Enable SSL: change the port to 465 and select SSL from the encryption drop-down menu
   (or Port 587 with SSL/TLS).
9. Save the settings.

Outlook 2013

1. Start Outlook.
2. On the Tools menu, click Account Settings.
3. Select the email account from the list and click Change.
4. On the Change E-mail Settings window, click More Settings.
5. Click the Outgoing Server tab.
6. Make sure the server name ends with "his.com".
7. Check the My outgoing server (SMTP) requires authentication option.
8. Click the Advanced tab and change the Outgoing server (SMTP) port.
   Enable SSL: change the port to 465 and select SSL from the encryption drop-down menu
   (or Port 587 with SSL/TLS).
9. Save the settings.