Some suggestions:

1.) Check that outgoing ports 25 and 587 (not sure which iRedMail uses) are open at machine firewall and router level if behind a  NAT, if you use port 465 make sure this is open too

2.) Check that have a DNS TXT entry with your SPF record

3.) Check that you have a static IP with a reverse PTR record and the corresponding DNS TXT entry
(Know for a fact that AOL rejects mail that fails a reverse PTR lookup...not sure about Yahoo or Google)

sorry problem is incomming...:( outgoing is fine.

Jun 19 02:34:00 mail policyd: rcpt=163, greylist=new, host=87.126.68.178 (unknown), from=hrvg@87-126-68-178.btc-net.bg, to=hr@example.com, size=0
Jun 19 08:34:00 mail postfix/smtpd[22138]: NOQUEUE: reject: RCPT from unknown[87                                                      .126.68.178]: 450 4.7.1 <hr@example.com>: Recipient address rejected: Policy Rejection- Please try later.; from=<hrvg@87-126-68-178.btc-net.bg> to=<hr@example.com> proto=ESMTP helo=<87-126-68-178.btc-net.bg>
Jun 19 08:34:00 mail postfix/smtpd[22138]: disconnect from unknown[87.126.68.178 ]
Jun 19 08:37:20 mail postfix/anvil[22140]: statistics: max connection rate 1/60s for (smtp:87.126.68.178) at Jun 19 08:33:57
Jun 19 08:37:20 mail postfix/anvil[22140]: statistics: max connection count 1 for (smtp:87.126.68.178) at Jun 19 08:33:57
Jun 19 08:37:20 mail postfix/anvil[22140]: statistics: max cache size 1 at Jun 19 08:33:57
Jun 19 08:43:05 mail postfix/qmgr[25838]: 92E5613A80EE: from=<>, size=3183, nrcpt=1 (queue active)
Jun 19 08:43:05 mail postfix/qmgr[25838]: CE79F13A8E7C: from=<root@mail.example.com>                                                      , size=2463, nrcpt=1 (queue active)
Jun 19 08:43:05 mail postfix/qmgr[25838]: D412913A8106: from=<>, size=4218, nrcp                                                      t=1 (queue active)
Jun 19 08:43:05 mail postfix/local[22455]: 92E5613A80EE: to=<root@mail.example.com>,                                                       relay=local, delay=13200, delays=13200/0.03/0/0.08, dsn=4.3.0, status=deferred                                                       (temporary failure)
Jun 19 08:43:05 mail postfix/local[22456]: CE79F13A8E7C: to=<root@mail.example.com>,                                                       relay=local, delay=18594, delays=18594/0.01/0/0.08, dsn=4.3.0, status=deferred                                                       (temporary failure)
Jun 19 08:43:05 mail postfix/local[22455]: D412913A8106: to=<root@mail.example.com>,                                                       relay=local, delay=17200, delays=17200/0.09/0/0.05, dsn=4.3.0, status=deferred                                                       (temporary failure)
Jun 19 08:34:00 mail postfix/smtpd[22138]: disconnect from unknown[87.126.68.178]
Jun 19 08:37:20 mail postfix/anvil[22140]: statistics: max connection rate 1/60s for (smtp:87.126.68.178) at Jun 19 08:33:57
Jun 19 08:37:20 mail postfix/anvil[22140]: statistics: max connection count 1 for (smtp:87.126.68.178) at Jun 19 08:33:57
Jun 19 08:37:20 mail postfix/anvil[22140]: statistics: max cache size 1 at Jun 19 08:33:57
Jun 19 08:43:05 mail postfix/qmgr[25838]: 92E5613A80EE: from=<>, size=3183, nrcpt=1 (queue active)
Jun 19 08:43:05 mail postfix/qmgr[25838]: CE79F13A8E7C: from=<root@mail.example.com>, size=2463, nrcpt=1 (queue active)
Jun 19 08:43:05 mail postfix/qmgr[25838]: D412913A8106: from=<>, size=4218, nrcpt=1 (queue active)
Jun 19 08:43:05 mail postfix/local[22455]: 92E5613A80EE: to=<root@mail.example.com>, relay=local, delay=13200, delays=13200/0.03/0/0.08, dsn=4.3.0, status=deferred (temporary failure)
Jun 19 08:43:05 mail postfix/local[22456]: CE79F13A8E7C: to=<root@mail.example.com>, relay=local, delay=18594, delays=18594/0.01/0/0.08, dsn=4.3.0, status=deferred (temporary failure)
Jun 19 08:43:05 mail postfix/local[22455]: D412913A8106: to=<root@mail.example.com>, relay=local, delay=17200, delays=17200/0.09/0/0.05, dsn=4.3.0, status=deferred (temporary failure)

Policy rejection is caused by greylisting, you should be able to receive this email in up to 30 minutes (maybe longer, but it will eventually arrive in your mailbox).

I don't understand what the real issue is.

Cannot deliver email to root user, do you have an alias for root user in /etc/postfix/aliases?

sir, actually problem is not getting incomming mail from others like gmail. yahoo etc.. but outgoing mail is wokring fine. this is the issue.

Subject    Undelivered Mail Returned to Sender
From    MAILER-DAEMON@mail.example.comAdd contact
To    root@mail.example.comAdd contact
Date    Today 03:56 AM
Delivery reportUndelivered Message
This is the mail system at host mail.example.com.

I'm sorry to have to inform you that your message could not
be delivered to one or more recipients. It's attached below.

For further assistance, please send mail to postmaster.

If you do so, please include this problem report. You can
delete your own text from the attached returned message.

                   The mail system

<root@mail.example.com>: temporary failure
Reporting-MTA: dns; mail.example.com
X-Postfix-Queue-ID: 41BFD13A80EE
X-Postfix-Sender: rfc822; root@mail.example.com
Arrival-Date: Thu, 18 Jun 2015 03:37:01 +0600 (BDT)

Final-Recipient: rfc822; root@mail.example.com
Original-Recipient: rfc822;root@mail.example.com
Action: failed
Status: 4.3.0
Diagnostic-Code: x-unix; temporary failure
Received: from localhost (unknown [127.0.0.1])
    by mail.example.com (Postfix) with ESMTP id 41BFD13A80EE
    for <root@mail.example.com>; Thu, 18 Jun 2015 03:37:01 +0600 (BDT)
X-Virus-Scanned: amavisd-new at mail.example
Received: from mail.ecampl;e ([127.0.0.1])
    by localhost (mail.example [127.0.0.1]) (amavisd-new, port 10024)
    with ESMTP id 4yVx+P327wG7 for <root@mail.example.com>;
    Thu, 18 Jun 2015 03:36:48 +0600 (BDT)
Received: from mail.example (unknown [127.0.0.1])
    by mail.example (Postfix) with ESMTPS id 0B39013A80DA
    for <root@mail.example.com>; Thu, 18 Jun 2015 03:36:48 +0600 (BDT)
Received: (from root@localhost)
    by mail.example (8.13.8/8.13.8/Submit) id t5HLU1Nx017870;
    Thu, 18 Jun 2015 03:30:01 +0600
Date: Thu, 18 Jun 2015 03:30:01 +0600
Message-Id: <201506172130.t5HLU1Nx017870@mail.example.com>
From: root@mail.example.com (Cron Daemon)
To: root@mail.example.com
Subject: Cron <root@mail> bash /var/vmail/backup/backup_mysql.sh
Content-Type: text/plain; charset=UTF-8
Auto-Submitted: auto-generated
X-Cron-Env: <SHELL=/bin/sh>
X-Cron-Env: <HOME=/root>
X-Cron-Env: <PATH=/usr/bin:/bin>
X-Cron-Env: <LOGNAME=root>
X-Cron-Env: <USER=root>

bzip2: Output file /var/vmail/backup/mysql/2015/06/18/mysql-2015.06.18.03.30.01.sql.bz2 already exists.
bzip2: Output file /var/vmail/backup/mysql/2015/06/18/vmail-2015.06.18.03.30.01.sql.bz2 already exists.
==> Backup completed successfully.
==> Detailed log (/var/vmail/backup/mysql/2015/06/18/2015.06.18.03.30.01.log):
=========================
* Starting backup: 2015.06.18.03.30.01.
* Backup directory: /var/vmail/backup/mysql/2015/06/18.
* Backing up databases ...
  - mysql [DONE]
  - vmail [DONE]
  - mysql [DONE]
  - vmail [DONE]
  - policyd [DONE]
  - amavisd [DONE]
  - roundcubemail [DONE]
  - iredadmin [DONE]
* File size:
----
121M    /var/vmail/backup/mysql/2015/06/18/amavisd-2015.06.18.03.30.01.sql.bz2
8.0K    /var/vmail/backup/mysql/2015/06/18/iredadmin-2015.06.18.03.30.01.sql.bz2
440K    /var/vmail/backup/mysql/2015/06/18/mysql-2015.06.18.03.30.01.sql
88K    /var/vmail/backup/mysql/2015/06/18/mysql-2015.06.18.03.30.01.sql.bz2
20K    /var/vmail/backup/mysql/2015/06/18/policyd-2015.06.18.03.30.01.sql.bz2
48K    /var/vmail/backup/mysql/2015/06/18/roundcubemail-2015.06.18.03.30.01.sql.bz2
276K    /var/vmail/backup/mysql/2015/06/18/vmail-2015.06.18.03.30.01.sql
44K    /var/vmail/backup/mysql/2015/06/18/vmail-2015.06.18.03.30.01.sql.bz2
----
* Backup completed (Success? YES).

Jun 19 11:41:35 mail postfix/smtpd[8810]: NOQUEUE: reject: RCPT from unknown[98.136.216.139]: 450 4.7.1 <mamun@example>: Recipient address rejected: Policy Rejection- Please try later.; from=<khan_cse2004@yahoo.com> to=<mamun@example> proto=ESMTP helo=<nm34-vm10.bullet.mail.gq1.yahoo.com>