Smart hosts/outbound SMTP is already included with just about all the mail filtering services out there. These days people like the features of having spam and viruses filtered before hitting their Exchange server. Also for us SMB consultants setting up our business clients with this service is both beneficial to them..as well as us, it's less maintenance...plus we repeat the montly income of the service.
Now, if your first rebuttal...if this client ONLY uses Exchange internally...meaning they ONLY send to other internal people and ONLY receive from other internet people..and ZERO e-mail comes in from the internet, and no outbound e-mail is allowed..then yes obviously clearly a service like MXLogic or Appriver is not needed. Uhm...that's not a question or point here...so true...it's not needed.
We still do the PRT/RevDNS 'n all that stuff for our clients...yes it mostly helps, but as you start taking care of business clients over time...you'll notice other things that can get them some drops in the servers reputation...so hey, if I can permanently provide a fix for that in a whole 30 seconds...why not? I don't like being bothered with some request for help from a client asking why their e-mail to some regular business contact of theirs keeps ending up bagged in their regular business contacts Barracuda.
We actually whipped up our own filtering/outbound services for our clients. Their MX records point to our main office for MX1, and then 2x offsite locations for MX2 and MX3...in case our office is offline. Mail goes through our multiple virus/spam filtering services...obvious mail dropped at the tar pit, rest of the mail flows through filters and the rest of the spam is put in a quarantine for the user. Quarantine digest sent to each users mailbox at 6am each day..they can access via web link and release and manage their own white list from there. Remaining "clean" mail goes through our forwarders and onto the clients public IP address...so the clients firewalls have port 25 only open/forwarded to our IP ranges. Also we have 3x outbound SMTP locations for their servers to utilize for outbound.
So for, on average, 250 bucks per year for the average SMB sized client, it's a good deal for them. And it makes it easy for us to manage their Exchange servers..it's a "global per domain" system...we don't have to add/manage individual users mailboxes like you do with appriver or postini.
That 250 bucks/year isn't really a waste of money, once you factor in the ease of setup, the ease of maintenance, the added security to the clients exchange server.
Also....we all know from experience...if their server is down..the default concept of mail queuing for them for approx 24 hours without some filtering service doing this for them is not really accurate. You're at the mercy of any/all the senders mail services acting independently.