SMTP Architecture

Hi
I was looking into setting up SMTP across multiple servers along with
a managed mail service and wanted feedback if there are any flaws with
the architecture I'm considering

1) All Incoming mail goes to the managed mail service
2) All manually generated (human) mail / replies etc use the managed
mail service
3) Each machine has an outgoing mail MTA that handles mail generated
locally (by various applications)
4) Incoming port 25 is blocked for the machines since they'll simply
be sending outgoing mail and won't be needing incoming connections
5) Publish MX Records indicating the managed mail service
6) Publish SPF Records indicating the local machines and managed mail
service as valid outgoing servers.

Are there any obvious gotcha's that I'm missing. Also are there any
good sites/forums etc with documentation related to best practices,
recommendations etc that I could read, wasn't particularly successful
with googling for those.

After setting this up, some time in the future I was thinking of
funneling all outgoing mail through the managed mail provider (with
the local MTA's serving as relays to it)

Thanks