Questgion:  Its easy to configure postfix to accept incoming email for multiple domains, but what happens if you have a client who wants their email SENT from a dedicated IP address?

Answer: Postfix 2.7.+ sender_dependent_default_transport_maps

sender_dependent_default_transport_maps (default: empty)

A sender-dependent override for the global default_transport parameter setting.
The tables are searched by the envelope sender address and @domain.

A lookup result of DUNNO terminates the search without overriding the global default_transport parameter setting.

This information is overruled with the transport(5) table.

Note: this overrides default_transport, not transport_maps, and therefore the expected syntax is that of default_transport.

This feature does not support the transport_maps syntax for null transport, null nexthop, or null email addresses.

For safety reasons, this feature does not allow $number substitutions in regular expression maps.