Add SPF record for Google Apps for your Domain

I mentioned before, I use Google Apps for your domain as one email service of my sites.
Email service switch is very smooth.
And user interface is same as Gmail, friendly.
One issue found. I send an email to an email address ended by
The Delivery Status Notification (Failure) message said:
Technical details of permanent failure:
PERM_FAILURE: SMTP Error (state 8): 550 MI:SPF mx20,wKjR5bDbf6h9JaNFxfU9DA==.29331S5 1168320069
The error code is 550 MI:SPF
That means the IP address of sender is not included in sender’s Domain list.
To check the SPF record, go here.
I checked Google help, the answer is here.
How do I set my SPF records?

Sender Policy Framework (SPF) records allow domain owners to specify which hosts are permitted to send email on behalf of their domains, making it hard to forge From: addresses. We strongly encourage you to publish SPF records for your domain — having these records in place will ultimately help fight spam.
To set your domain’s SPF record, publish the following TXT record on the DNS resource: v=spf1 ~all.

Updated 2010-7-22
v=spf1 ~all