1) Welcome to the Party!
2) As you are obviously new here, I have some suggestions to help us all help you:
When you post, please post your entire configuration including (but not limited to) your installation method (7.X.X?) and vicidial version with build (VERSION: 2.X-XXXx ... BUILD: #####-####).
This IS a requirement for posting along with reading the stickies (at the top of each forum) and the manager's manual (available on EFLO.net, both free and paid versions)
You should also post: Asterisk version, telephony hardware (model number is helpful here), cluster information if you have one, and whether any other software is installed in the box. If your installation method is "manual/from scratch" you must post your operating system with version (and the .iso version from which you installed your original operating system) plus a link to the installation instructions you used. If your installation is "Hosted" list the site name of the host.
If this is a "Cloud" or "Virtual" server, please note the technology involved along with the version of that techology (ie: VMware Server Version 2.0.2). If it is not, merely stating the Motherboard model # and CPU would be helpful.
Similar to This:
Vicibox X.X from .iso | Vicidial X.X.X-XXX Build XXXXXX-XXXX | Asterisk X.X.X | Single Server | No Digium/Sangoma Hardware | No Extra Software After Installation | Intel DG35EC | Core2Quad Q6600
3) "it completely make my server crash" is a bit vague. Are you saying it turned off? MySQL stopped running? Postfix service failed? Hard drive filled up?
4) Have you configured postfix aside from the default installation of a Vicibox install? Or is this a non-Vicibox installation? (If it's a non-vicibox install, this question is Entirely unrelated to this forum!)
5) Whitelist lock down your server. Your Vicidial server should not be visible to the world and should not be accepting email from anyone via postfix. Vicidial servers are meant for outbound email only. Any inbound email is handled a different way, not through postfix at all.