In what situations might the error "Warning: mail() [function.mail]: SMTP server response: 553 We do not relay non-local mail, sorry" occur, and how can it be addressed effectively?

This error occurs when the SMTP server does not allow relaying of emails that are not from a local domain. To address this issue effectively, you can set the 'sendmail_from' directive in your php.ini file to a valid email address from your domain.

ini_set("sendmail_from", "valid_email@yourdomain.com");