You can change the zimbraMailTransport property for the user that it's delivering messages to, but Zimbra is really overkill for a project like this
It's something like:
Code:
zmprov ma user@domain.com zimbraMailTransport smtp:mail.domain.com:25
That will make Zimbra deliver messages destined to that user to the SMTP server you specify.
That can also be set on a per-domain level as well.