Can I open an application someone sent me by e-mail?

Always use caution when opening (such as by double-clicking) files that come from someone you do not know, or if you were not expecting them. This includes email attachments, instant messaging file transfers, and other files you may have downloaded from the Internet. Any time that you download from a source that has not previously earned your trust, you should take extra precautions. This is because a downloaded file might have a name or icon that makes it appear to be a document or media file (such as a PDF, MP3, or JPEG), when it is actually a malicious application. A malicious application disguised in this manner is known as a “Trojan.”

How can I distinguish legitimate and malicious applications?

Where you got the file is the most important indicator. Only download and install applications from trusted sources, such as well-known application publishers, authorized resellers, or other well-known distributors. It is also advisable to use antivirus software to scan any files before installation.