1. Keep it SimpleWhen contacting someone by email, especially if they don't know who you are, the best thing you can do is keep it simple. Explain who you are and how you are related to the person or family you are contacting them about. Briefly explain your interest in family history. If appropriate, pass on greetings from the relative or researcher who has put you in touch. Don't overwhelm the recipient with questions or your entire family tree in this first email. Consider it an introduction to your conversation.
2. Make the Subject Line CountPeople are so swamped with email these days, especially spam, that they often scan the subject lines for important emails and just delete the rest unopened. Subject lines such as "hello" may just not be opened. I often include the full name or surname of the specific ancestor that I'm writing about. If the individual you're emailing shares the same family name, however, be sure to include words like genealogy. A subject line such as "Mascarelli Genealogy" is more likely to get someone's attention than a simple "Hello."
3. Breaking the Language BarrierIf the person you're contacting does not speak your language, then see if you can find someone who does to help you compose your email. If you're not sure, then you may want to send the email in both languages - your native language and theirs.
Language translation sites can help in a pinch. Use simple words and phrases in your email to help increase the chance that they will be translated correctly. Check and correct key facts such as names and dates before you send. Sometimes they may be translated as well!