iRule to allow by source mac address ?

This iRule works for me

when SERVER_CONNECTED {
    clientside { TCP::collect }
}

when CLIENT_DATA {
    set hostOK RK
    set payload [string toupper [TCP::payload]]
    log local0. "payload=$payload"
    if { $payload starts_with "EHLO" } {
        log local0. "EHLO found"
        if { $payload contains "EHLO $hostOK" } {
            log local0. "SMTP OK, is from $hostOK*"
            TCP::release
        }
        else {
            log local0. "SMTP ERROR, $payload is NOT from $hostOK"
            TCP::respond "550 Hostname denied\r\n"
        }
    } else {
        log local0. "EHLO not found"
        TCP::release
        TCP::collect
    }
}