PHP Mail Function Not Working (Not Received Mail For User)

Im using Webmin+Virtualmin to run my Xenforo forum.But now i have some problems with mail system.
Then after registering users on my forum system will sending conformation mail for user email.(Confirmation mail Sent!)
But it did not received to user mail account.(Google,Yahoo…)

My System Information :
Operating system :CentOS Linux 7.4.1708 (Linux 4.4.96-mainline-rev1 on x86_64)
Postfix Mail Server : Installed & Running (Postfix version 2.10.1)