Take the 2-minute tour ×  Possible Duplicate: Outgoing mail from
linux not being delivered  As someone marked this a a duplicate
I cannot post the actual answer that worked for me and that can
be found here: http://serverfault.com/a/128218/42894 (and not in
the duplicate link).  Our website email system based on the
CakePHP (1.3) email component has been working perfectly fine
for a year and suddently it's not sending emails since yesterday
to certain domains anymore, and sometimes even randomly. I use
the mail() php function to send emails via the website (account
modifications, project updates, ...), and we retrieve our emails
via gmail (we are using google apps). When I try to send an
email to a user in our own domain, I get the followin error in
/var/log/mail.log:  Mar 9 10:12:29 newserverxxxx sm-mta[27999]:
q29ACTlt027999: < user@ourdomain.com >... User unknown  We also
have issues with dsn=4.0.0, stat=Deferred on certain users from
other domains receiving randomly their emails or not.  We are
running a