This alert on an SMTP check on Mxtoolbox is trivial.
It could be because your outbound (sending) mail server is blocking un-authenticated port 25 use by spammers. Instead you can authenticate (login) on port 487 to send email.
This from Mxtoolbox:
“Also, the banner mismatch is just a warning. There’s nothing that says that having a mismatch (or no hostnme in your banner at all) will cause you to have difficulty sending or receiving mail, it’s really more of a best practice.”