When you create a new email in the three-pane layout of .Mac webmail, a separate window pops up for your email composition. Make sure you don't navigate to another site from the main .Mac webmail page while you're writing your email. If you navigate outside of webmail with that window, your message will appear to send but will not actually be sent.
Be sure to complete your email message and click send while the mac.com site is still in the main window. If you need to view a different webpage while composing your email, open a new window for the different webpage.