I didn't find any way to set postfix with spamd to check outgoing email from webmail/localhost for attempt to ward off possible spam.
The spamassassin daemon is not itself a Postfix content-filter (SMTP proxy), nor a milter for directly processing email content.
If you want to use spamassassin as a content filter, install amavisd-new instead of spamd, and configure Postfix to send whatever mail you want through it and pick up the returning email. See www.postfix.org/FILTER_README.html.
It's a less common or supported approach, but if you want to use spamd as a milter, look at the SpamAssassin wiki and choose a wrapper for spamd like MIMEDefang or milter-spamd and look at www.postfix.org/MILTER_README.html.