linux - Sendmail: Use SMART_HOST for local mail

We are running an outbound SPAM filter, and we'd like to be able to have the mail sent through that filter, even when it's to a local user. Is there a way to force sendmail to use the SMART_HOST, even when it is going to be delivered locally?

Example

Here's what I want. My host is example.com. If I send an email from me@example.com (using SMTP) to you@example.com, I want the email to go from mail.example.com to outbound.example.com to inbound.example.com, then back to mail.example.com. Is this possible without creating an endless loop?

Answer

This sounds like a job for procmail as the LDA. Use it to invoke your spam filter for any locally generated email.

define(PROCMAIL_MAILER_PATH',/usr/local/bin/procmail')
FEATURE(local_procmail_lmtp)
MAILER(procmail)
MAILER(procmailnoalias)dnl

and in your global procmail rule, look for an x-header not present to then send the mail through for scanning

e.g
:0H
* < 10000000
* !^X-Locally-Scanned: MySuperScanner