If you are using WebRatio 6.1.0 you can change this property selecting the SMTP server node and look in the Properties View (refer to the following image) .
If you are using a previous version, you can change this information adding to the Mail Unit a Property having "mail.smtp.port" as name and the port you want to use as value. The Mail Unit, in fact, supports all the properties available from the Java Mail API. Refer to http://javamail.kenai.com/nonav/javadocs/com/sun/mail/smtp/package-summary.html for the full list of properties.