1. Impersonate another person (via the use of an email address or otherwise) or otherwise misrepresent yourself or the source of any email.
Don't use someone else's Gmail account and write emails pretending like you are him or her. According to Time, there are nine states with online impersonation laws, and in New York and California online impersonation is a misdemeanor punishable by thousands of dollars in fines and up to a year in jail. So just don't create fake Gmail accounts in the first place. Ahem, Lennay Kekua, cough cough.
2. Send emails to users who have requested to be removed from a mailing list.
If you are still receiving emails from an address you unsubscribed from, try the troubleshooting advice in this Google blog post: unsubscribing made easy. Google launched a service that messages senders on your behalf, asking them to unsubscribe you from their list. Unfortunately it doesn't work for all senders right now, but Google is encouraging senders to support auto-unsubscribe.
3. Sell, exchange or distribute to a third party the email addresses of any person without his or her knowledge and consent.
The going rate for 1 million U.S. email addresses is $25, making your personal email address worth $0.000025. Besides the fact that selling email addresses without the person's knowledge or consent is illegal, it's not very lucrative. Trying to make a quick buck? Don't do it this way.
4. Send unsolicited emails to significant numbers of email addresses belonging to people you don't know.
Not only is selling email addresses illegal, but sending emails to a bunch of people you don't know is also illegal. So no, you cannot email blast the cast of Downton Abbey to ask about Matthew Crawley and Lady Mary's nuptials, or where Dan Stevens is eating dinner tonight...
5. Send, upload or distribute any unlawful, defamatory, harassing, abusive, fraudulent, infringing, obscene or otherwise objectionable content.
Email fraud has become rampant as technology continues to develop, but sending harassing or obscene content is also illegal. The U.S. government has drafted up a helpful manual for recognizing fraudulent content with tips on what to do when you encounter it. This list of illegal content seems pretty all-inclusive, so just send, upload and distribute only cute pictures of cats, okay?
6. Conduct or forward pyramid schemes and the like.
In order to avoid inadvertently becoming part of a pyramid scheme, check out some helpful tips. The old adage "If it sounds too good to be true, it probably is" works well in this situation. (Sorry, Bernie.)
7. Transmit content that may be harmful to minors.
In email and in life, ya heard? That includes child p**nography, correspondence of a threatening, violent or graphic nature and age-restricted content, such as alcohol, drug or dating site material.
8. Illegally transmit another's intellectual property or other proprietary information without such owner's or licensor's permission.
Go to Tumblr; it's called a "reblog." Just make sure you have permission before sending anyone's work through the Gmail service, or else face the consequences.
9. Interfere with other Gmail users' enjoyment of the service.
GIF courtesy of loverofeverythingsweet
10. Sell, trade, resell or otherwise exploit for any unauthorized commercial purpose or transfer any Gmail account.
If you can't sell your account in the first place, how can you resell it? Besides that point, you can't make money from your domain name — even if it is really, really cool.
11. Use the Gmail service in connection with illegal peer-to-peer file sharing.
If you are illegally downloading music, movies, or TV shows (tsk, tsk), don't use the Gmail service to share the content with your friends. But remember: Friends don't let friends illegally peer-to-peer file share.
12. Modify, adapt, translate or reverse engineer any portion of the Gmail service.
Reverse engineering involves reproducing a manufacturer's product using a detailed examination of its construction. Basically, you can't take the structure of Gmail, copy it and then create your own email service.source: