I keep getting all these emails with strange attachments from people I don't know. Are these viruses?
What you are seeing - strange emails with files attached, and returned emails that you did not send - are the symptoms of many email-based PC viruses.
If someone with a PC is infected with one of these email based viruses (like Klez) and has your email address on their computer, the virus can grab your email address and use it as the sender when it emails the virus to the rest of the collected addresses. This virus type has the ability to auto-send itself from a computer, by using this computer's address book. The virus can also pretend to be anyone contained within this address book.
But only on PCs.
This means that a friend or relative of yours, or anyone who'd have your e-mail address within his/her address book has been infected by the virus, and the virus either arrived in your mailbox or sent itself pretending to be you (your e-mail address). This person is also using a PC.
We'd like to point out that VirusBarrier does not scan emails within your mailbox, you must first save or open any attachment onto your hard drive to enable VirusBarrier to detect and eradicate the virus within the saved/opened file.
But when you come across an email which has the generic characteristic of an email-based PC virus (curious subject wording like "How are you?", or attachment with double extensions ".html.pif" for instance), you should simply delete the email, without bothering to save the attachment.
If VirusBarrier finds one of these viruses in any of your files, it'll delete the file. Luckily, the files which come with these emails are not executable/usable on a Mac, but the deletion takes place to prevent Mac users from emailing the infected file(s).