A full implementation of SMTP protocol allows the ability to perform mail routing bypassing DNS MX records by means of setting, in a ruled way, the 'RCPT TO: <>' request. A mail from 'xuser@hostz' directed to '@hosta,@hostb:foouser@hostc' is received by '@hosta' then sent to '@hostb' using 'MAIL FROM: <@hosta:xuser@hostz>' and 'RCPT TO: <@hostb:foouser@hostc>'. The message is then sent to '@'hostc using 'MAIL FROM: <@hostb,@hosta:xuser@hostz>' and 'RCPT TO: <foouser@hostc>'.