SmtpCommandException when sending mails

Oh, in addition, we're using version 9.2.

Have you tried setting it like the following:

Common\MailNotification\TransportSecurity = None

Wait I see that you had it set to none and it did the same thing.

Did you set the SMTP Relay Parameter? 

Can you post the parameters of the failed sendmail job?

Yes, I set it, in fact the string "<SMTP Relay>" that I inserted in the error was just a placholder to the real value of the parameter.

The parameters are:
- AcceptSelfSignedCert: ""
- Account: ""
- Address: "user.test@domain.com"
- AllowServerNameMismatchInCert: ""
- AuthenticationString: long encrypted string
- BaseObject: "<Patch Display="Test, User (USTE)"><Key> etc. etc. "
- ConnectionProvider: "VI.DB.ViSqlFactory,VI.DB"
- ConnectionString: long encrypted string
- Domain: ""
- Encrypt: ""
- EncryptionCertificateScript: ""
- MailID: "CCC - Employee - End date has changed"
- Password: long encrypted string
- SenderAddress: mail of the sender
- Sign: False
- SignCertificateThumbprint: ""
- SmtpPort: 25
- SmtpServer: name of the SMTP Server
- TransportSecurity: "None"
- UseDefaultCredentials: "False"

I would try a connection to that MTA using simple smtp commands , or either an external email client, and try to send mails via that MTA, so you could discard , isolate and locate the origin of the problem: SMTP server (MTA) or OneIdentity.

Seems to me (as the error message offers no other info) the server is refusing to relay mail depending on the sender/recipient domain or ip address (mta based problem).

Hello,

at last it was a server issue. Unfortunately I don't know which issue. The customer just gave me another server and that worked.

Thank you everyone for the help,
Lucrezia