Admin - Email settings - Optional user/password not working

I have updated the Admin → Email settings with our SMTP server and the Sender address leaving the optional user/password fields blank.

When I click on the ‘Test SMTP Server’ button I receive an error notification:

server: 535 5.7.0 authentication failed

When I populate the user/password fields the test succeeds.

I have tested sending email with this smtp server without auth, using telnet, and it does work.

Has anyone else experienced this error when updating your Email settings and not using the optional username/password? If so, were you able to solve the issue?

Using Corteza 2023.3.0

Thank you.