If you believe that the message was identified as spam in error, then you (or another moderator that you appoint) can approve that message to be posted to the group. You can do this several ways.

From the Group Home page:

• The number of pending spam messages will appear in the Management Tasks section. Click on Spam Messages Waiting For Review link, which will take you to the Pending Spam Messages page.
• Review, edit, approve or delete the pending spam messages by clicking on the appropriate link.

From the Messages section:

• Click on the Spam? link in the navigation under Messages. You’ll see a list of messages that were identified as spam (if any exist).
• Review, edit, approve, or delete the pending messages by clicking on the appropriate link.

From email:

If you’re receiving notifications of pending messages (available in the Edit Membership area of a group), you’ll be notified via email every time there is a message awaiting your approval. From the email you can approve or reject the message.