Paul Cocker wrote:
Under the existing setup the postfix (secondary MX) doesn't deliver mail
internally, it passes it on to the Barracuda. The idea being that should
the Barracuda fail we can allow temporary internal delivery from this
server but for 99% of the time manage all mail via a single interface.
The postfix box does run recipient checks on incoming mail but that's
it.
The problem I see is that once the Barracuda bounces a mail forwarded by
postfix with an SPF failure or RBL block we generate backscatter because
Barracuda tells postfix that it's not taking that mail and postfix then
sends a bounce notification.
The general solution is to configure all MXes identically (at least with
respect to mail they'll accept for relay) - whether they then hand off
messages to each other for whatever reason is entirely up to you.
If you're "just" rejecting based on SPF failures or fairly standard RBL
processing, you should be able to set up both of these on your Postfix
box without too much hassle. I would personally recommend against
rejecting based on too many RBLs; there are really only a few that IMO
are safe to reject on outright.
If the Barracuda appliance rejects on message content, with per-user
customization.... it gets "interesting". With a recent enough Postfix,
you may be able to get a milter of some kind working to reject mail by
holding the SMTP session open while the milter attempts to relay the
message to the Barracuda box.
-kgd
