Crystal, from Germany, asks: "A stranger sends an email with an attachment for a photo/file what do you do?"

Hi Crystal! Here's a short answer: Don't open any attachments from people you don't know. Now for the slightly longer answer...

There are certain types of files which are relatively safe to open, but even ones that you think would be safe might surprise you. Here's a Q&A that talks about one of those: File Attachments and Viruses. Even if you felt confident that a file type was safe to open, you need to be aware that people can be very sneaky. A file which is named "myimage.jpg?x=5.exe" might fool you into thinking it's a JPEG image file, but actually, because it has ".exe" on the end, it's an executable (program) file. You don't want to be opening that!

So if a complete stranger sends you some sort of attachment on an e-mail, you do one thing, and one thing only: you delete that email without opening the attachment. 

Unless...

Unless you work for a business where you might expect to receive e-mails from strangers. For example, suppose you work for a corporation in their personnel department. It's not unreasonable to suppose that you will get e-mails with resumes attached, from people who want to work for your company. I know this happens, because even though I've never advertised that I want to hire someone to work for me, I often get those kinds of e-mails.

So maybe they're legit. But maybe they're not. What to do?

Well, there are a few possibilities.

1. Make sure you have the latest greatest virus software installed, cross your fingers, pray, and open that word processor document (Microsoft Word has a feature which causes documents from the internet to be opened in an uneditable "safe" mode, which makes it less likely that the file will give you problems, but I won't suggest that it decreases the probability to zero).
2. Contact the person and tell them that you don't accept electronic resumes, and...
   1. Ask them to mail you a printed copy.
   2. Give them a fax number, and ask them to fax the document to you.
3. Contact the person and ask them to put it into Google Docs, and share it with you.

Obviously, all but the first of these options result in hassle for everyone involved, so most people wil choose option 1. And under certain business circumstances it's impossible to avoid opening attachments from strangers. So if you're in a business situation where you have to open docs from strangers, make sure your virus software is up-to-date!

By the way, you can call me paranoid, but I'm even cautious about opening attachments from friends. After all, what if their computer is infected with a virus? Then their attached file could be infected as well, and they wouldn't even know it! It's like having chicken pox - you are contagious even before you know that you've been infected.

I get file attachments from both friends and strangers. Should I just open the ones from friends?

I would even think twice before opening file attachments from friends. What if your friend's computer has been infected by a virus, and the virus emails you a message with itself attached? You get the email, think it's from your friend, and open the attachment. *Poof!* Your computer is now infected!

The only time you should open an attachment even from a friend is if it is a file type that can't carry viruses (generally speaking you can open image files, but even with those, believe it or not, someone developed a virus to attach to an image (read below for a question and answer about that).

If you have any doubt about whether a file type can carry a virus, don't open the email. Call or email your friend and say "Did you send me this file?" If they say no, obviously you aren't going to open it. And if the message is from a stranger? Don't open it at all! I occasionally get resumes from people who want to work for me, and rather than mailing them, they send them as Microsoft Word Documents. I don't know them from Adam, and I won't open their attachments.

I get emails with picture files (stuff with .gif?x=5, .bmp, and .jpg?x=5 file extensions) attached. I know there are some kinds of email attachments I should never open; what about these? Can I get infected through these?

Well...yes, technically, it is possible, but only under the following scenario. Your computer has to already be infected with another virus whose purpose is to act as interpreter for the image virus. Seems kind of pointless to me. (The "Perrun Virus" was an example of this kind of virus, which attached to jpg files. A lot of hype, not much action)

However, you still need to be careful when opening images attached to emails. Be sure that the file extension really is an image extension (the ones you listed, plus a few others). I've seen emails come in with file names like "yourpicture.jpg?x=5.exe". Clever. The extension isn't really 'jpg', it's 'exe', and you already know you should never open those. And remember: if the attachment is from someone you don't know, why would you bother opening it anyway?

Is it okay to put my credit card number, expiration date, and other information in an email? I'd like to purchase a product online, and they want me to put my credit card info on their website; is it really any different to put it in an email?

Actually, this question is the one that finally convinced me it was time to create my Q&A website. I was absolutely horrified yesterday to receive an email from a potential customer who wanted to buy one of my products, and had put his credit card number, expiration date and name in his email to me! Never, never, never do this! Don't even think about it! People tend to think that email is a private correspondence between them and their intended recipient. That would be a nice theory, but it is not guaranteed!

Remember that emails do not send or receive over secure sockets, which means if someone was looking and analyzing in the right place at the right moment, they could intercept your data packets and snag your credit card number. Where your credit card number is concerned, it is better to be paranoid than sorry.

The emails I get from my friends have all kinds of odd puncutation marks in them; a colon followed by a close-parenthesis, or a greater-than sign, followed by a colon, followed by an open-parenthesis. These obviously mean something to the people who sent them, but they don't mean anything to me. Can you help?

Your friends are using something we call "emoticons", or "emotional icons". They are symbols which represent a particular facial expression. The best way to interpret them is to imagine them as faces tipped sideways. So if you look at this one - :), and imagine it rotated 90 degrees, you've got a smiling face, with the colon as the eyes, and the parenthesis as the smile. Here's a quick "crash course" on internet emotion indicators:

:) = smile
:D = laughing smile
:( = frown
>:( = angry (the greater-than symbol is downturned eyebrows
::) = rolling of the eyes*
:P = tongue sticking out
:-/ = sad/uncertain/mixed emotions
B) or 8) = someone wearing sunglasses "cool"
:O = surprise
;) = wink
:-[ = sad

* The eye-rolling emoticon used to be very popular, but I hardly ever see it any more.

I have been getting undeliverable emails sent back to me to email addresses that I do not recognize. In the last three days I have received emails from two individuals asking me if I was a spammer. Can you please tell me what the heck is going on??????

Imagine 3 people A, B, and C. A knows B, and B knows C, but A and C do not know each other. In this scenario, B has both A's and C's email addresses stored on his computer. Now person B gets a virus on his computer, and the virus creates an email message, randomly grabs two email addresses, puts one of them (A) into the "from" line, and the other (C) in the "to" line, and sends the email off.

Now person C gets an email from A, a person he doesn't know, and assumes A is a spammer. He sends a nasty email to A saying "are you a spammer?" That answers the second part of your question. To answer the first part, imagine that person C has changed his email address, so the email address the virus sends to is invalid. So what happens? Since the virus has forged A into the "From" line, it is A who gets the "delivery failure" notifications, even though he never sent out an email. Is there anything you can do about it? Not a thing. Sorry.

Somebody, maybe more than one somebody, is faking my email address and sending out potentially illegal spam. You said to grin and bear it when somebody forges your email address but I don't want the authorities knocking at my door over this. Is there any new technology to help me out?

Remember that every email has a path it travels, which can be found by looking at the "options" for the email. For example, the email you sent me has the following path: [Email information snipped] Now, that may look like a lot of garbage to you (it certainly does to me) but the fact is, once the email leaves the forger's computer, he has no control over that information. So the "authorities" could always tell whether or not this really came from you. You only have something to worry about if you really are the sender.