Yes, you are correct, the trial (free or trial Gapps accounts) have a limit of 500 messages a day, or 100 if you're using an application.
Google support: Gmail email send limit - no more than 500 recipients per message for the Gmail web interface, or max 100 recipients if you are using an email client software.
However, if you upgrade to a full Google Apps account, then the limit raises to 2000. Many other restrictions are removed as well.