acl_check_ismx:

  accept  spf         = pass

  # Use helo if null sender

  warn    senders     = :
          set acl_m1  = $sender_helo_name

  # If not use sender's domain

  warn    !senders    = :
          set acl_m1  = $sender_address_domain

  # Find MX servers

  warn    set acl_m1  = ${lookup dnsdb{>: defer_never,mxh=$acl_m1}}

  # If no MX servers try hostname

  warn    !senders    = :
          set acl_m1  = ${if eq {$acl_m1}{}{$sender_address_domain}{$acl_m1}}

  warn    senders     = :
          set acl_m1  = ${if eq {$acl_m1}{}{$sender_helo_name}{$acl_m1}}

  # Lookup hostnames

  warn    set acl_m1  = ${lookup dnsdb{>: defer_never,a=$acl_m1}}

  accept  hosts       = $acl_m1

  deny