Greylisting is a technique used to block bad mail clients of the sort used as spambots before they have a chance to dump their load into our mail server.
Most spammers don't have the luxury of keeping track of all the emails they have to resend and just move on to their next target. This is because sending email takes resources, and delaying and resending takes more resources than it's worth to them: their objectives is to dump as much spam as quickly as possible before their become blacklisted. Their window of opportunity is quite narrow.
Greylisting can be configured to let through some recipients or some servers that we know are friendly.
At any rate you should not lose any mail because of greylisting: any mail sent through a RFC compliant (normal) mail server will end up being delivered.
There are ready-made packages that come already preconfigured:
# yum -t postgrey
# chkconfig --levels 235 postgrey on
This will install in your
To find out the format of the rules, check the content of
Any non-matching email will be at first rejected for a few minutes so its sending server can try again to submit it through.
Setting up postfix
Postfix needs to be made aware of postgrey or it won't use it.
Then restart postfix and the postgrey services:
# service postgrey restart
# service postfix restart