The two best ways to reduce the amount of unwanted email in your inbox is to report spam messages and unsubscribe from mailing lists you are not interested in. Gmail will try to automatically unsubscribe you from messages you mark as spam.
The open nature of the global email system is a double-edged sword. By nature it allows any person or company to communicate with anyone else. The drawback is that sender's do not always have good intentions. Even if the message appears to be from someone you trust, you should always question the sender's intentions.
Malicious messages can come from anyone. It's very common for an attacker to hijack an account, and then use that account to send harmful messages to all of the contacts. Even if a message is from someone you know, be on alert if it seems unusual.
Emails which attempt to persuade you to do something dangerous often use language which adds urgency so that you are more likely to suspend your suspicion and perform the action anyways. Vague messages which implore you to open some link or attachment are frequently malicious. Examples of dangerous actions are:
Naturally, if you never trusted any message or opened any attachment, email would not be very useful. Use your best judgement and if you feel a message is unsafe, you may want to report it as a phishing or spam. Reporting spam and phishing emails is good practice, because you will help Gmail reduce the amount of unwanted messages you receive. If a message is probably safe, but you do not want to receive any more message like it you may be able to unsubscribe from marketing messages from that sender.
A spam email is generally an unsolicited marketing message. Spam may or may not be malicious. It is dangerous to open any links or attachments in spam messages unless you are quite sure the message is safe. When you report a message as spam, the message will be submitted to the Gmail team for analysis and the message will be moved to your spam folder. By reporting spam, you help Google improve their spam filtering technology for everyone and you can reduce the amount of junk mail in your inbox.
Often marketing messages will give you the option of unsubscribing from their list. If the “spam” message is from a company you've previously dealt with, you may want to remove yourself from their mailing list by unsubscribing. According to the CAN-SPAM Act, email marketers are required to provide you with an option to unsubscribe from their messages. You should apply this method only to messages in your inbox from companies you recognize. Messages in your spam folder are more likely to contain malicious links.
It is always safest to mark messages as spam. Especially if:
Read the message and search for an unsubscribe or update subscription link—usually at the bottom of the mail. Visiting that link will allow you to opt out of future mailings from that company.
A phishing email is a malicious message that attempts to convince a user to divulge sensitive information. By reporting phishing messages to Gmail you help them improve their filtering so that fewer people receive malicious messages. When you report a message as phishing, it will be removed from your inbox and placed in your spam folder.