Outgoing Mail (SMTP) Servers usually restrict who can send mail through them. Most ISPs restrict the use of their outgoing mail servers to IP addresses that it owns.
If you are getting this error, you are probably connecting to your Internet Service Provider (ISP) and trying to use a our outgoing (SMTP) mail server.
You need to put your ISP’s Outgoing Mail (SMTP) Server in your mail programs preferences. Leave our incoming (POP) server in there in order to check mail, but if you want to send mail set the outgoing SMTP server to your ISP’s designated mail address. Contact your ISP directly and ask for their SMTP configuration.