No basic test for mailbox availability? (Very obvious false valid)

clearout.io

Hi,
I just tried a test email and was surprised that it returns valid, even though a very basic self written test would identify it as invalid, as the mailbox is not available.
Why does't this easy one fail and report accordingly?
Best

Array ( [0] => test.testmann@t-online.de )
Array
(
    [mx01.t-online.de] => 10
    [mx02.t-online.de] => 10
    [mx03.t-online.de] => 10
    [mx00.t-online.de] => 10
    [t-online.de] => 0
)
try mx01.t-online.de:25
<<< 220-mailin87.aul.t-online.de T-Online ESMTP receiver fssmtpd2025 ready.
220 T-Online ESMTP receiver ready.
>>> HELO test123456.de
<<< 250 Ok.
>>> MAIL FROM: <user@test123456.de>
<<< 250 2.1.0 Sender accepted.
>>> RCPT TO: <test.testmann@t-online.de>
<<< 550-5.1.1 user unknown
550 5.1.1 Unknown recipient.
>>> RSET
<<< 250 2.0.0 Ok.
>>> quit
<<< 221-2.0.0 mailin87.aul.t-online.de closing.
221 2.0.0 Closing.
test.testmann@t-online.de invalid.