Confirm the email address before you hit send!
Nowadays, it's not uncommon for people to have multiple email addresses. Some people even belong to group email accounts in which an email sent to one address is actually received and potentially read by multiple people.
Before you hit send, be sure you know exactly where your email message is headed. Even when you're replying or forwarding, take the extra moment to hover your mouse over the address in the "To" field to be sure it's going to the intended address.
If you find yourself making this mistake often, consider changing email clients. Gmail, for instance, is notorious for allowing this recipient confusion. Gmail users should also be aware that Google has copies of and access to all email sent using its system. Mr. Snowden provided some proof of that.
Businesses especially should always use a proprietary domain for their email (not Gmail, Yahoo, etc., and certainly not a social email address, like those from Facebook). Business owners should always ensure their email provider follows good security practices (e.g., not storing any email on their servers after it is delivered to the client destination).