Can SMTP be configured for each site in Multisite?

Hi,
I have a number of subsites that have contact forms and other features that generate emails. I want the emails to be sent from the subsite's domain rather than from the root site's domain. As i understand it, the default is that there is a single SMTP config for the whole network. How can I get each subsite to send its email from its domain? As currently configured, they end up in spam...
Thanks,
Neil