Let me describe this situation

I want to setup "Outgoing mail configuration".
Let's suppose my params are:
SMTP hosts: mymail.mycompany.com
SMTP security: None
SMTP Auth Type: PLAIN

With those settings, going to "Test outgoing mail configuration" I am able to send a test email.
I am happy.

In spite of this I found that even if
SMTP Auth Type: PLAIN
(and not SMTP Auth Type: LOGIN)
if I set a dummy SMTP username and password, the test email is no longer delivered reporting the error: "Your site couldn't communicate with your mail server. Please check your outgoing mail configuration.".
Why do SMTP username and SMTP password are used even if (SMTP Auth Type != LOGIN)?

Is this a bug?

I didn't go to check for the code but, IMO, I think that something like

if ($CFG->smtpauthtype != 'LOGIN') {
} else {
}

should craft the email server connection string.