RW wrote:

quoted text
>

> The others were from bots as far as I could tell but they were not

> being sent by MTAs which had received them.

> 

Yes, but the OPs problem is back scatter, and that does not come from

bots, they don't retry.
$ man spamd:
DESCRIPTION

      spamd is a fake sendmail(8)-like daemon which rejects false mail.

      It is designed to be very efficient so that it does not slow down

      the receiving machine.

..

..

      greylisted hosts are redirected to spamd, but spamd has not yet

      decided if they are likely spammers.  They are given a temporary

      failure message by spamd when they try to deliver mail.
Greylisting works brilliantly for bots, but wont help with hosts that

retry, as is the case in back scatter.
If the OP was repeatedly getting mail to a few addresses from different

hosts, he could use grey trapping. But he said that they are all random.