How do suppression lists work?

Regarding point #1, yes, unfortunately leads on a suppression list are counted against your database limit (unless you were able to negotiate this during contract signing.

Suppression lsits are counted against our database limit. Most of my static lists are in my leads database and I store them in the My Lists folder. The suppression lists I name according to the type of email campaign. I do an unsubscribe trigger campaign.  If someone unsubscribes from a particular email campaign, they are added to a static list that's specific to that email campaign.

I have suppression lists for the following:
Event type
subscription fulfillment by publication
trial subscription fulfillment by publication
trial subscription promotion by publication
blog alerts
podcast alerts
bookstore promotion
publication promotion
advertising promotion
sponsorship promotion
online database promotion
general announcement
customer doesn't want to receive any email

You can create a suppression list for absolutely anyone who doesn't want, or shouldn't get, your emails. Here are some ideas (by creating a smart list):

• Email Address contains "@competitordomain.com"
• Inferred IP address is "competitorIP"
• Email Address is empty
• Email Invalid is true
• Unsubscribed is true
• Company Name contains/is
• Inferred Company contains/is

Then, when you're sending emails, exclude the smart list from your campaign. Alternatively you can create a static list, but I would be wary of the possibilities of someone becoming someone who should not be on your suppression list.

Have fun :)

Thanks for the input everyone. Sounds like we've got some data cleanup to do on our end.

I especially like the idea posed in the thread you shared Josh. I hope it happens soon!

Great tip Josh.  Also I just spoke with someone from Marketo yesterday because our database has recently shot up and he suggested creating a smart list of those leads with no interactions from the last 18 months and decide whether to suppress or purge.