SMTP response 221 appears to generate an SMTP invalid response alert
We have a public facing mail server and we see a lot of SMTP invalid reply alerts when it issues a response like:
530 5.7.0 Must issue a STARTTLS command first
221 2.0.0 Bye
Now, I suppose this is indication of a failed attempt to send mail through it, but it’s not really an “invalid reply” and it’s not unexpected.
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