- The base OS may be using postfix (or one of the others) for alerts.
- The services you are running may be using one of those services as an MTA.
I'd advise turning them down, one at a time. Just stop one service. Then watch your logs for an hour, and test the other services. If you start seeing errors, you know something is configured to use the mail service you turned down. Check logs frequently for a day or a week. If none of your services complains, you can disable the service. When you're ready to uninstall, be sure to capture the configurations for the service, in case you need to restore it.
Repeat the full cycle for each mail service. It is important to only stop one at a time. Also, keep in mind that any problem that shows is most likely caused by the latest one you stopped, but it's possible that it was caused by an earlier service shutdown.
Also, before you start turning down services, grep through the config files for all services and cron jobs, core OS and non-core, looking for the mail services by hostname and IP address. (Actually, this may not be productive. Since you are not talking about a dedicated mail server, the server names will be there for other services.) Read through the installation and configuration instructions for the non-core services you're running, with an eye toward the configuration steps that deal with MTAs.