As anyone that uses email will know, UCE/junk mail is coming in thick and fast.

One quick setting in postfix ( presumably you can do this with other smtpd servers but I will only show postfix )
that cut down the amount of UCE arriving at my inbox was the postfix main.cf entry:

smtpd_recipient_restrictions =....<snip>check_helo_access hash:/etc/postfix/helo_checks  

Add that entry to your main.cf file then create an ASCII (text) file called helo_check with content similiar to below.

localhost                       REJECT You are not me
123.123.123.123                 REJECT You are not me
host.example.com                REJECT You are not me

Don't forget to issue a postmap on the helo_checks file.
Any mail server helo'ing to your server now with your ip, hostname or localhost will be rejected before its even allowed to send you mail/UCE.

Mar  9 04:50:20 scrappy postfix/smtpd[28556]: connect from 122-124-139-172.dynamic.hinet.net[122.124.139.172]
Mar  9 04:50:22 scrappy postfix/smtpd[28556]: NOQUEUE: reject: RCPT from 122-124-139-172.dynamic.hinet.net[122.124.139.172]: 554 5.7.1 <123.123.123.123>:
........................Helo command rejected: You are not me; from=<hi7188s.pp5975@msa.hinet.net> to=<zz@mail2000.com.tw> proto=SMTP helo=<123.123.123.123>
Mar  9 04:50:22 scrappy postfix/smtpd[28556]: lost connection after RCPT from 122-124-139-172.dynamic.hinet.net[122.124.139.172]
Mar  9 04:50:22 scrappy postfix/smtpd[28556]: disconnect from 122-124-139-172.dynamic.hinet.net[122.124.139.172]

Whilst I have no exact figures as to how much UCE this has stopped I can say it has made a noticeable difference.