The Infomail filter specializes in identifying and filtering newsletters or similar ad-focusing emails. Treating such emails differently from regular spam makes sense, as newsletters are not necessarily unwanted by some recipients.
You can find the Infomail settings in the Control Panel under “Security Settings> Spam & Malware Protection> Generel Settings> Activate infomail filter”. To enable the filter, just select the checkbox and klick "Apply changes" afterwards.
Once enabled, you can decide whether the infomails should be quarantined or marked. Should you choose to mark them, you can add an appropriate phrase or word from the text field, which will be added to the subject of the email.
The filter is activated by default for all users. You can disable the filter for each user individually though. You can find this setting from the user preferences at “Filter & Reports”.