When a mail client connects to a mail server to receive or send mail, authentication is used for security purposes. There are different authentication methods. Some of the options are CRAM-MD5 or APOP, which are currently outdated and insecure. Here's how to check your current settings and set up a more modern and secure authentication method.
In Outlook 2016/2019 for macOS, manual configuration is not required. The application determines the appropriate authentication method automatically.
To properly set up authentication in Outlook 2016/2019, do the following:
Open the section "
File»:

On the tab “
Intelligence»Click«
Server parameters»:

Check your inbox settings:

Expand and check the outgoing mail settings:

Save your changes and test your mail.
To properly configure authentication in Outlook 2013, please do the following:
Open the section "
File»:

On the tab “
Intelligence»Click«
Account setup…»:

Select the desired mail account and click "
Edit…»:

The parameter “
Secure Password Checker (SPA)"- a check mark opposite it
necessarily must be removed. If the checkbox is checked, uncheck it. Then click “
Other settings ..»:

Check the settings on the “
Outgoing mail server»:

Switch to the "
Additionally"And check the encryption settings:

Save your changes and test your mail.