90.000 incoming spam delivery failures a day - how do I defend myself?

Question

My server gets 90.000 delivery failure notifications daily.
A spammer has decided to use random fake "from" adresses from my domain and send their junk to non existing adresses all over the world.
They don't end in my inbox because postfix rejects them. For each attempt it leaves 3 lines in my log files like this:
Nov 23 14:01:48 echoone postfix/smtpd[26561]: connect from relay.milio.com[213.33.179.194]
Nov 23 14:01:48 echoone postfix/smtpd[26561]: NOQUEUE: reject: RCPT from relay.milio.com[213.33.179.194]: 550 <[email protected]>: Recipient address rejected: User unknown in local recipient table; from= to=<[email protected]> proto=SMTP helo=<relay.milio.com>
Nov 23 14:01:48 echoone postfix/smtpd[26561]: disconnect from relay.milio.com[213.33.179.194]
This makes my mac mini slow when I send mail, and I think that bad things will happen if it gets worse.
Can anybody beat my 90.000 incoming rejected spams a day - and is rejecting this with postfix sufficient?
It has been going on for weeks - and I did not notice it until I dug into why my server took 15 seconds to send even tiny emails.
Why do they do this?
Non existing email adressees don't read spam - nor do I !!!
Good they don't use my real email address - yet.
Henrik
Mac Mini G4   Mac OS X (10.4.8)  

Answer 1

A spammer has decided to use random fake "from"
adresses from my domain and send their junk to non
existing adresses all over the world.
This unfortunately is common practice. It's not that they send to non-existing adresses only. It's just that you only get the bounces for the non-existant ones, since he used a from adress from your domain.
Most spammers just use harvested e-mail addresses. They do not pay the money for proper e-mail databases that have to some extent been verified. What counts are the numbers. If they send 10'000'000 mails, they couldn't care less if 90'000 are non-existant (would be different if they had to buy stamps .
They don't end in my inbox because postfix rejects
them.
Which is good and unfortunately about the only thing you can do to avoid these mails end up in your queue.
This makes my mac mini slow when I send mail, and I
think that bad things will happen if it gets worse.
Nothing bad happens because of rejected e-mails per se.
The only thing to consider is that a mini (and in particular its harddisk) are not designed for the high and constant level of I/O operations of a mail server.
Can anybody beat my 90.000 incoming rejected spams a
day - and is rejecting this with postfix sufficient?
Not only is it sufficient, but there isn't really anything else you can do. Even if you use other means of rejection like access list or spam filters, it's still your server being hit. So it is best to reject at the postfix level.
At least in a one server environment, I don't see what else you could do. A firewall or router can only reject at the IP level, but since you say these mails are bounces, they will come in from thousands of different IPs.
Not the answer you wanted, I know. Just the harsh reality of the internet. It is time for digital signatures to prevent sender spoofing, but it is not gonna happen for quite some time unfortunately.
Alex