Recently having done some email troubleshooting I am reminded of the importance of Google Apps users adding a DNS TXT record for SPF to prevent your Google Apps emails from possibly getting tagged as spam. See this entry in Google Apps Help.
To set your domain’s SPF record, you should have access to your domain’s DNS settings. On your DNS resource, publish the following TXT record: v=spf1 include:aspmx.googlemail.com ~all
And here is another link to a page in Google Apps Activation Guide that has a bunch of links to how to create the relevant DNS settings for several popular web hosts, such as GoDaddy.
Detailed Instructions for Specific Domain Hosts