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This article outlines the important difference between evaluating a message as spam vs phishing.
Spam
Spam is unsolicited junk email sent indiscriminately in bulk, often for a product or service. Much of spam is sent by networks of virus-infected computers (known as "botnets"). According to various estimates, about 80% of all emails in the world may be spam. While annoying, spam is in the vast majority of cases both legally permissible and not a security concern.
For these reasons, it is rarely worthwhile to report or trace the origins of a spam message. It is most efficient to simply mark such messages as spam and/or block them.
Phishing
Phishing may look like spam, but its real purpose is to defraud users into giving up their credentials, passwords, credit card numbers, and other valuable personal information. Phishing emails will often tempt you to click on links that redirect you to fake websites. They often pretend to be from organizations such as a bank, PayPal, or Amazon. This behavior is always criminal, and never safe!
Never respond to any unsolicited email asking you to provide personal information. Reputable organizations and businesses will never reach out and ask for such information in this manner.
It's best to report phishing emails as Phishing (not Spam) in Gmail.
While reporting as either will cause Google to analyze and adjust their filters, reporting as Phishing does something extra - with enough reports, that phishing email gets a banner applied to it, warning other recipients to proceed with caution. It's a nice value-add, but one that doesn't get triggered if reported as Spam.
More information
You can read more about phishing in Google's documentation.
To report an email as spam or phishing, please refer to the following knowledge base article: Reporting spam and phishing via Gmail.