Yes you are right to any email "inside" sis.at.com I can send email but if I try to send it to an external email like padawanlinuxero@gmail.com is giving me the same error message how can I fix this problem can you help me? Thanks
If your SMTP server requires authentication to send to outside hosts then you can't using Mail::Sendmail. If you read the documentation (the section entitled LIMITATIONS) it specifically says it doesn't support SMTP auth. Switch to another mail sending module which does support authentication, or bribe and/or coerce your mail admin into letting the source box in question send mail to outside hosts without authentication.
If it's a linux box, it probably has sendmail locally. That's the SMTP server you'd use.
If it's Windows you'd probably use a remote SMTP server to relay your traffic. Check the config of a working email client and you'll probably see how it's handled in your environment.