A few nights ago I received an email from "2CO" asking me to update my personal data. The sender did not forget to insert a link to log in, too.
Following are the steps that I have usually taken to discover whether an email came from a rightful person/company or a swindler:
Position the mouse pointer above the link provided by the sender(PLEASE DO NOT CLICK IT!). See on the status bar whether the URL that appears is genuine.
If you use Outlook Express, in the message list, select the suspected message. click "File", and then click "Properties". Click "Details" to see the email headers.
Notice the "Return-path" part, where you will see the sender's original email address. Notice the domain name (e.g ...@xyzdomain.com). Now you can guess with more confidence that it is a real or fake email.
If you have ever received an email from an autoresponder, you might have noticed that the "Return-path" part contained the domain name of the autoresponder (e.g. ....@abc.getresponse.com) although the email address looked like from another domain.
If you are still not sure, do the same with the original email which you had received previously (the one from the right person/company) for comparison. If the header of the original email on the "Return-path" part reads "Return-path: ", while on the suspected email the reading is "Return-path: ", you ought to be suspicious that the second is a fake email!
If you want, you can also notice other parts of the header such as "Received-from" and "Message-id". But, please do not be influenced by the "from" part. It is the part that is usually used to manipulate you.
Additionally, please conceive that an organitation which is always cautious about malicious abuses, such as 2CO, ClickBank, etc, will hardly send a link to you to click in the email. As I have experienced personally, if the company has something to inform me, they merely ask me to log-in to my account (There is no link to click).
Dispatching a faked up email can be done very easily by anyone who possesses a little knowledge about programming (php, etc).
I hope this tip will be useful for you.