ahmedsa
As I & others have said, changing the mx record is only part of your problem, you have other problems.
I tested your smtp server & you have a problem.
SMTP Connect Failed To Connect More Info
Session Transcript:
Connecting to 78.93.244.61
11/18/2015 7:44:45 AM Connection attempt #1 - Unable to connect after 15 seconds. [15.05 sec]
11/18/2015 7:45:05 AM Connection attempt #2 - Unable to connect after 15 seconds. [15.00 sec]
PWS3v2 35069ms
Also if I open a web browser at
http://mserver.altawi.com.sa/I get
ERROR
The requested URL could not be retrieved
The following error was encountered while trying to retrieve the URL:
http://mserver.altawi.com.sa/Connection to 78.93.244.61 failed.
The system returned: (111) Connection refused
The remote host or network may be down. Please try the request again.
Also if I open a web browser at
http://78.93.244.61/I get
ERROR
The requested URL could not be retrieved
The following error was encountered while trying to retrieve the URL:
http://78.93.244.61/Connection to 78.93.244.61 failed.
The system returned: (111) Connection refused
The remote host or network may be down. Please try the request again.
DanB35's test on your open ports is suggestive that you do not have required ports open & forwarded to your SME server eg 25, 80 for starters, & more depending what services you run etc.
Something appears to be changing this situation/setup, as you say sometimes "it" works & sometimes "it" does not work.
You can do a test from a workstation that is behind your SME server
ie
a workstation on the LAN that is using your SME server as a gateway,
go to
http://www.grc.com& run a port scan to see what ports are open.
I think you previously said your SME server is in server only mode & you have a Cisco firewall in front of it, is that correct ?
Check the Cisco firewall to see what ports are open & see if they are forwarded to your SME server.
If you do not know how to do this then you need to get an IT professional to help you.