Yes, I do exactly this for custom applications I have created for clients. These applications generate many emails, ranging from ecommerce receipts to digital tickets to exam reminders.
The way I have configured it is to have a single subdomain for mail relays, and then I create one account for each client application. Each account then has a strong randomly generated password for SMTP authentication to the mail server. For instance,
client@relay.host.com. I've found that this gives me a lot of flexibility in configuration and I can easily determine if one of the accounts is being abused. I can get reports of email volume per client as well. I don't know what kind of volume you are looking for, but I do hundreds of thousands of emails per month.
Michael's point about deliverablity is right on. I do set up SPF and DKIM, but I have to be extremely proactive to monitor our server's reputation. We do occasionally have problems being blacklisted, despite our emails being legitimate and not marketing-related. It seems that due to the volume of emails we send, even a small number of reports triggers listings, so be prepared to fight those battles with Microsoft, Comcast, Yahoo, etc. And, I have to think that some people just click the spam button (instead of the delete button) by mistake, not realizing that it will wreak havoc for email system admins for the next week. Sign up for their feedback loops and monitor any reports. Check out
www.senderscore.org.
The other problem I face is limiting downtime while updating the email server software or operating system. I find that to be annoying. Most applications that send email don't have a backup SMTP connection to try, and most don't use a queue to store or re-try mail. So, if customers are making purchases during the time we're updating the software, their email receipt won't get sent. (In newer application we are writing, we are trying to address this by making each email into a task, and then retry the task if sending it fails.)
You could look at SendGrid (Twilio) as an alternative. I use Twilio for SMS notifications and like their service a great deal.