I want to share how to solve the problem if our server (I use Redhat base) suddenly cannot sending the email.

1. You can check the email log with code
   

   tail -f /var/log/maillog

2. In My case, I have problem :
   [code] postfix/smtp[22031]: F2F76411B9: to=<———–>,
   relay=aspmx.l.google.com[00.00.00.00]:25, delay=1129,
   delays=1127/0.07/0.9/1.3, dsn=2.0.0, status=sent
   (250 2.0.0 OK 1392263109 ix4si518951pbb.29 – gsmtp)
   Feb 13 10:45:09 webPrimary postfix/qmgr[22023]:
   F2F76411B9: removed[/code]

   When you check the result, this problem caused no resolve.conf (In my server, I use postfix app for sending the email)

3. Check the file resolve .conf with command :
   

   cat /etc/resolv.conf

4. Because of use google apps for email sending, I put the DNS google to the configuration
   [code]nameserver 8.8.8.8
   nameserver 8.8.4.4
   [/code]