You've heard that you shouldn't open attachments unless you know who they're from. Or click on links in e-mails without first checking the "tooltip" (the little message that appears when you hover your mouse over the link).
But when you're a member of an e-mail list that is using the program called "Mailman," there is sometimes an “attachment” that looks like “ATT00242.txt (349B)”. When you try to open it, you can't. That's because this is the way some e-mail systems handle the e-mail list footer, that is, the part that says:
Your website mailing list
You can safely ignore any “attachment” that looks like that.
"Mailman" an ad-free program CMwD and many others use to host their mailing lists rather than using a group system that sends advertisements to its group members. It is part of a website hosting package and not all website hosts supply this program. If you have any more questions about this type of list, contact your web hosting provider.