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.
There is a way but it is such a stupid thing that I only provide the steps.

Put this into your recipient\_restrictions: `recipient_restrictions = ..., static:HOLD, ...`
Then you will get an output of `postqueue -p` where all mails in hold-queue are marked with an `!` (exclamation mark) and the arrival date. Use a cron-job and a script to parse and examine this output. If a mail is longer than x minutes in the queue you can reinject it to the Postfix delivery process by issuing a `postsuper -H [queue_id]`. The [queue\_id] is in the first column of `postqueue -p`.

And please don't ask for the script to automate this!