Google acquired Postini email security services years ago, and now it is integrated with Google Apps service. If you configured your domain name using Google Apps services in the past using Postini, you need to update your domain name MX records.
Using Postini service, you configured your domain name MX record something like this. I'm using KING.NET domain name as an example.
@ (none) KING.NET.S9A1.PSMTP.COM. 10
@ (other) KING.NET.S9B1.PSMTP.COM. 10
@ (other) KING.NET.S9B2.PSMTP.COM. 10
@ (other) KING.NET.S9A2.PSMTP.COM. 20
The new MX records listed below.
Blank or @ 300 MX 1 ASPMX.L.GOOGLE.COM
Blank or @ 300 MX 5 ALT1.ASPMX.L.GOOGLE.COM
Blank or @ 300 MX 5 ALT2.ASPMX.L.GOOGLE.COM
Blank or @ 300 MX 10 ASPMX2.GOOGLEMAIL.COM
Blank or @ 300 MX 10 ASPMX3.GOOGLEMAIL.COM
Excerpt from Google email "Your email continues to flow through Postini systems until you change the MX records to point directly to Google for all of the domains listed above. Until you change MX records, your email relies on Postini systems and this makes a more complicated mail flow."
Who's affected? I know my Google Apps for Business domain name I need to update the MX records pointing to the new information as listed above. I don't need to update the MX records for my domain names using Google Apps (free version).
I'm not sure for domain names using Google Apps for Education.
(Source: 2012, Whaddya.com)