On our AWS EC2 instance email delivery stopped working without any notice. Customers started reporting that they are not receiving their emails. Better check your error logs.

If you find something like this.

PHP Warning: SimpleEmailService::sendEmail(): Sender – RequestExpired: Request timestamp: Sun, 15 Sep 2013 06:39:50 UTC expired. It must be within 300 secs/ of server time.\nRequest Id: 552d8117-1dd2-11e3-a1bc-29ded7e8e9e2\n

This basically means that the timestamp of our server is off and does match the Amazon Simple Email Service. It is off by almost 300 secs.

By why will our server time suddenly be off. It appears that server time needs to be synch to a central NTP server. You can try the following suggested solutions.

1. Update locally
Install the ntpdate package on your system.
#sudo apt-get install ntpdate
#sudo /usr/sbin/ntpdate 128.101.101.101

2. Run and check against existing NTP Servers
#sudo /usr/sbin/ntpdate 0.north-america.pool.ntp.org 1.north-america.pool.ntp.org 2.north-america.pool.ntp.org 3.north-america.pool.ntp.org

Kudos to Mind Geek for the second solution.