Question: is there a way to configure delay with which emails will be processed by postfix?
What I'm aiming for is e.g. 5 minute lag between receiving mail from MUA and transferring it to mailbox or other MTA.

Thanks.

Answer: 
This functionally should be implemented in the MUA. If you have a webmail you can install a plugin implementing this function. There is an extension available for Thunderbird. If a user sends an email and later changes her mind then she can cancel herself. I see no other way. Making nasty hacks on the server will just make things worse if one does not know all the answers already.
You could pretty easily do this by specifying a new master transport in the smtpd section of main.cf:

```
smtpd_recipient_restrictions = check_policy_service unix:private/sleep_delay, permit_mynetworks, reject_unauth_destination

```

Then in master.cf:

```
# ==========================================================================
# service type  private unpriv  chroot  wakeup  maxproc command + args
#               (yes)   (yes)   (yes)   (never) (100)
# ==========================================================================
sleep_delay  unix  -       n       n       -       -       spawn
        user=nobody argv=/usr/bin/perl /path/to/your_script

```

In /path/to/your\_script, just have it sleep(X) and exit(0). You can use this script/config as an example of how to implement external script interaction with postfix:

<http://www.howtoforge.com/postfix_spf>