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Since you can see who the sender is from your inbox list without having to open the message, you can deduce if a message is spam by simply looking at the sender’s email address. That said, some spam and phishing scams will pretend to be major companies, so you can't assume that an email from "Amazon" is guaranteed to be non-spam.  If the message was sent from a website that you don’t recognize or an email address from someone you don’t know, chances are the message is spam. In rare cases, spammers control other people's accounts, meaning you may get emails from your "friends" who have been hacked. Checking the sender is the first, not the only, step you should take. If the sender's address has a bunch of numbers or a domain you don't recognize (the part after the "@") then the email is likely spam. You likely already know most of these -- sales, investment opportunities, new treatments, requests for money, sex, information on packages you never ordered, etc. Usually, you are being offered something, often for nothing. If you didn't order it, don't assume you forgot. This is simply a scam tactic to make you click a bad link. If you want even more specifics, the US FTC has a list of the 12 most common spam email types on their website. This is known as phishing, when a criminal pretends to be a reputable site, like PayPal, that has to "update user information," or needs you to sign in "immediately." In general, if the email asks for immediate action or personal information, it is phishing and should be ignored. One of the most common subject lines, "Problem with your Account" is almost always phishing. If you had a problem, it will tell you when you log on to the account. For example, hover your mouse over the following link for www.google.com. Don't click -- instead, look to the bottom-left corner of your screen, where a different URL (one for Wikihow) shows up instead of Google. Spammers do this trick all the time to bring you to dangerous sites. Be especially wary if the address is a set of numbers -- most reputable companies will use words instead of numerals. Spam can come from all over the world and can be written in various writing forms to pique your interest. Find any misspelled words One of the ways spam can get past a filter is by rearranging letters of words that spam filters look out for.  For example, spam might spell the word “sexual” as “sxeual" to avoid being picked up. You might see this in URLs as well, like sending you to "Paypal" instead of PayPal, or www.ebay.random.words.and.numbers.10002122.com.  Spam normally contains extensive, large images that occupy most of the message body. The text is usually large to capture your attention. If you don't know the sender, can't trust a link, or otherwise feel like an email may be spam, do not open any attachments. This is the quickest way to a virus, bar none. If you must open the attachments, right click on them first, then select "scan for viruses" or "scan" before opening. Gmail automatically checks attachments for viruses, but it isn't perfect. The most common way you're information is compromised is by clicking a link in the spam email. However, if you're unsure if the email is legitimate or spam, you still have options. For example, if you get a packaging email you weren't expecting from Amazon, log on to Amazon and type in the order number to check it -- don't click on the "track package" link in the email. If you're still on the fence, there are some sites that let you check links before you've clicked on them. You can try getlinkinfo.com to see if there are a lot of "redirects," which likely means there is spam coming from the site. You can also use the program SiteCheck, which takes any URL and checks if there are malware or viruses on the page. Most email services have an anti-spam feature that filters suspicious messages and diverts them to a specific folder in your email account labeled “Spam.” If the mail server detects a message to be spam, it separates it from your other messages into the Spam folder, away from your inbox. This is the first and most obvious sign of a spam email.

Summary:
Ensure you know and trust the sender before opening an email. Check the subject line for common spam topics. Avoid any "calls to action" or requests for personal information. Hover over any links in the email to see if they match their supposed destination. Look for typos, especially of key phrases or words. Never open or download attachments unless you know what they are. Type in any links directly instead of clicking on the links. Use 3rd-party security sites to test emails and links you're still worried about. See if the message was diverted to the spam folder.