Note#1 : This can be used with SIMS v1.7 or above, though it's strongly recommended to get at least 1.8b8 from ftp://ftp.stalker.com/ -- it's stable, and has numerous additional features. This FAQ is written for 1.8b8 or above.
Note #2: This assumes one is using the HTTP administrative interface, rather than connecting with CommuniGator.
Step #1: Log into your SIMS mailserver with an account with administrative priviliges (usually postmaster).
Step #2: Select the "SMTP" tab from the left menu.
Step #3: Select the "RBL Server List" link in the bottom right.
Step #4: Enter bl.spamcop.net "See http://spamcop.net/bl.shtml" into the text field, and push "Update". (Notes in quotation marks are included in the bounce message and can be customized to each server. I've included a suggested wording.)
Step #5: Select the "SMTP" tab again from the left menu.
Step #6: Tick the "Use Blacklist DNS Server(s)" option in the bottom-center, and click "Update".
Thanks to Pete Stephenson for these instructions.