There is another problem, after reinstalling Webmin & VirtualMin the real memory usage is 128/128 and there is no free memory in real memory. Is it a risk to have several sites in this case?

You need to read the Virtualmin on Low Memory Systems guide here:

http://www.virtualmin.com/documentation/id,virtualmin_on_low_memory_systems/

Thank you andreycheck & joe for your comments,

I have tried many ways to change the value of net.ipv4.ip_no_pmtu_disc but all return an error about permissions. Now I have contacted with provider and waiting for their answer. No hope.

regards,

Hi Guys,

After more researching on the web, the problem has been fixed. I update this thread to make a further reference and for those who may have a similar problem.

Q: Why is it impossible to update the value of ip_no_pmtu_disc in sysctl (in my case)?
A: Because I am on VPS (virtual Private Server) and it is not allowed by default. (My researches showed to me that OpenVZ does not allow it)

Q: What was the problem really and how did I overcome the problem?
A: I am not expert but I would like to share my experience as a newbie. Actually the problem was where SendMail or Postfix were unable to send e-mail messages completely and in brief they did not finish the messages and sending tasks. While the remote servers were waiting for task completion but no thing were being done by our server. At last the remote servers were closing the connection (after no activity from our server). After researching on it I have found that it is related to MTU size. So I have changed the MTU size to 1000 and now Postfix works as a charm.

I hope the problem does not come back. If it comes back, I will come back here. :-)

Please edit my post if it contains false info.

Thanks guys