It is a good idea to mark messages that are a bit (or more) off-topic in the Subject: header already, so that people not interested can filter them to low-priority folders or ignore them altogether.
Use "OT" in the Subject to Indicate Off-Topic Messages
Usually, the keyword or acronym "OT" is used to indicate such messages:
- Put "OT: " (without quotes) in front of your message's subject.
For example, a message might come with the subject: "OT: My cat loves spinach!"