sender rewrining advice

       receive
        email
          |
          |
          |
          |
          V
   +------------+       +------------+      +------------+
   |     MX     |       |  OUTGOING  |      |     MX     |
   |            +------>|            |----->|            |
   |  accessmap |       |            |      |  EXTERNAL  |
   |            |       |            |      |            |
   +------+-----+       +------------+      +------------+
          |
          |
          V
   +------+-----+
   |    LOCAL   |
   |            |
   |  virtuser  |
   |            |
   +------------+
I would like to forward messages to external email addresses and apply sender rewriting. I don't have any experience with this, and was wondering what a default best practice is.
- on the mx server I want to decide what messages are for local delivery and what go to external.
Normally I have to first relay the message to a local host, where in the virtualuser table I have an entry to deliver to an email address.
I prefer to skip this. What could I use on the MX host? LDAPRoute?
- I prefer the messages to be routed via the 'OUTGOING' service
Because the MX are not specified in spf records. Assuming that such envolopes 'SRS0=HHH=TT=example.org=alice@example.com' are still being checked on spf.
- on the 'OUTGOING' I only have dkim signing
I guess best would be to first do some routing and then on the 'OUTGOING' do the sender rewriting. Anyone already doing something like this?