Once you install iRedMail on your own VPS you might encounter issues by mails not being received on time or not being received at all. In my case I just had to disable the Postfix Policyd greylisting. I have seen many reject error messages in /var/www/mail.log :

Jun 11 18:13:55 ProcessingEngine postfix/smtpd[13562]: connect from gateway01.websitewelcome.com[69.41.242.19]
Jun 11 18:13:56 ProcessingEngine postfix-policyd: rcpt=16, greylist=abuse, host=69.41.242.19 (gateway01.websitewelcome.com), from=ORIGIN@EMAILREMOVED.COM, to=DESTINATION@EMAILREMOVED.COM, size=0
Jun 11 18:13:56 ProcessingEngine postfix/smtpd[13562]: NOQUEUE: reject: RCPT from gateway01.websitewelcome.com[69.41.242.19]: 450 4.7.1 : Re cipient address rejected: Policy Rejection- Please try later.; from= to= proto=SMTP helo=
Jun 11 18:13:56 ProcessingEngine postfix/smtpd[13562]: disconnect from gateway01.websitewelcome.com[69.41.242.19]

After a bit of a research I have figured that the postfix-policyd configuration file contains the entry where you can disable greylisting. Once I have done that I have been able to send and receive mails instantly.

The config file on my ubuntu 10.4.2 LTS server is located at:

/etc/postfix-policyd.conf

Just set the GREYLISTING paramter to 0

#####################################################################
#
# enable greylisting                                  default: on
#
#   whether greylisting should be enabled or disabled.
#
#                                                     1=on  0=off
GREYLISTING=0