[CentOS] centos email server suddenly much slower. What to do?

[Les Mikesell]

On Thu, Sep 6, 2012 at 12:14 PM, Marco Fioretti
<marco.fioretti at gmail.com> wrote:
> Greetings,
> I run my own email server for some domains I administer, on a centos
> vps server with a very small number of users.
>
> The only services are smtp, imap/pop, webmail
>
> Everything was running without problems until this morning. I left
> home for 1/2 hours, and when I came back everything had become about
> 100x slower (seriously!). The services were/are still all up and
> running, but practically unusable (even running mutt in my ssh session
> is almost frozen).
>
> I have not changed/updated anything in the last 1/2 weeks. I have
> already done a few checks with the VPS provider, and it looks like:
>
> network & hardware are OK
>
> there seem to be no strange processes running. I didn't manage to save
> the output of "top", but it didn't show anything that (AFAICT, of
> course)  should not be there on an email server
>
> however, there is something that is using "much more memory than
> normal" (see the comment below from the hosting sysadmin after he
> checked user_beancounters). Initially we thought it was apache, but
> even switching it off didn't change anything. What now?
>
> Any help to understand what the heck happened, and find out what
> exactly _started_ to cause this problem is very welcome!

One thing to check is that the DNS servers in /etc/resolv.conf are
answering quickly  (dig some_name.domain @server_ip).  Mail services
use DNS extensively and if the first server fails there is a timeout
before trying the 2nd choice.  Things will still work but slower and
you may end up with enough processes running to run out of RAM and
start swapping.   Also check your outbound mail queue in case some
spam attempt has succeeded in generating bounces.

-- 
   Les Mikesell
     lesmikesell at gmail.com