for Postini for Google Apps
About MX Priority
DNS servers usually contain multiple prioritized MX records. MX records are prioritized using a numbering system. The priority helps specify which mail server your email is directed to. Having multiple mail servers and associated MX records is helpful when one server is down for update, repair, replacement, or malfunction, or if a company routes its email to different global departments within the organization. Prioritization helps specify which mail server routes the email in the event that one server is out of commission. Therefore, if one mail server is down, email can be routed to another mail server with a lower priority.
MX records with a lower numeric value have a higher priority than MX records with a higher numeric value. For example, an MX record with a priority of 1 has a higher priority than an MX record with a priority of 4. However, different domain hosts may use different priority numbering systems. Some domain hosts use a system where the top priority starts with 1, while others use a numbering system that starts with 100 — with lower priorities of 200, 300 and 400.
Some hosting services may use fewer than the four MX records you’ll receive while activating Postini for Google Apps. These services may also use a priority system of High, Medium, and Low; or Primary and Secondary, rather than a numbering system.