E-mails are permanent, that is they will be stored somewhere until deleted, usually by the recipient. It is much cheaper to send an E-mail than a letter. It is much cheaper than a long distance phone call. It saves paper and raises productivity within the office environment. Messages are usually delivered within minutes of being sent.
Many people only read mail first thing in the morning.
Be aware that some mail servers only poll for new mail at intervals, say one hour, which may cause a delay in the recipient getting his mail. This allows the recipient to sort incoming messages. E-mails are easily traced to source, and can be easily printed out and used against you. Try not to use Upper case throughout your message. This reflects badly on you and the company.