#main.cf
header_checks = pcre:/etc/postfix/header_dependent_relay

#/etc/postfix/header_dependent_relay

/^From:.*specialsender1@example.com/       smtp1:[host1.example.com]
/^From:.*specialsender2@example.com/       smtp2:[host2.example.com]
/^From:.*specialsender3@example.com/       smtp3:[host3.example.com]

#master.cf
smtp1    unix  -       -       -       -       10       smtp
    -o smtp_sasl_password_maps=hash:/etc/postfix/smtp1.relay
smtp2    unix  -       -       -       -       10       smtp
    -o smtp_sasl_password_maps=hash:/etc/postfix/smtp2.relay
smtp3    unix  -       -       -       -       10       smtp
    -o smtp_sasl_password_maps=hash:/etc/postfix/smtp3.relay

[hostX.example.com]   userX:passwordX