We talked earlier about using the "BCC" field when you send an email to a large group of people. If you use the "To" (or "CC") field, and if any one of the recipients has a compromised computer, then the email addresses are visible to any kind of malware.
Using the "BCC" field hides the email addresses from each other -- and from any malware on a comprised computer of any of the recipients.
But, there's another reason to use the "BCC" field -- though, to me, this isn't as important as the malware security issue, I think it's important enough.
If you use the "To" or "CC" field, you're making everyone's email address visible to everyone else. Not just the malware I mentioned earlier, but the actual eyes and email programs of everyone on the list.
Why is this a bad thing? Well, it might not be. But, are you sure that everyone on the list knows everyone else? And are you sure that everyone on the list is okay with everyone else knowing their email address?
Why would someone car? Actually, it doesn't matter why anyone would care. The fact that someone could care should be enough. I mean, do you give out phone numbers willy-nilly? You shouldn't. And neither should you give out email addresses without permission.
To some degree, it's a security issue, but not necessarily a malware issue. Rather, it's a matter of being polite.
So, I strongly suggest that, if you send an email to a large group of people, put your own email address in the "To" field, and the group's email addresses in the "BCC" field. It's good manners, and it helps prevent others from getting spam.