Public facing lists can often be the target of spam emails. Your mailing lists automatically have some default spam protection, but spam is unfortunately designed and altered to get through these filters.
On the Manage Email Lists page, on the "email settings tab" there is a field called "spam threshold" - by default it is set to 5. This is the highest spam score that the filters will let through. Some legitimate emails (especially other mailing lists, or corporate emails) may have a spam score above 0. if you're experiencing a high volume of spam, it's recommended to switch the threshold down to a number between 1.5 and 1.
It's also a good idea to enable moderation (see below) in order to have more control over which emails get sent ont o their recipients.