Unfortunately it is also a great way that hackers can infect your computer. They can send fake cards to you that , wen opened, cause you lots of problems. Fortunately these are easy to avoid. If you don't know the sender avoid opening it. If you think it may be legitimate look for things like misspellings in the message. If the message asks you to follow a link to download the card it is almost certainly a scam.
Another choice is, if you don't recognize the name of the card company, Google it to see if there are warnings or complaints.