From Lunarpages Web Hosting Wiki
Email is the electronic equivalent of postal mail though much faster. What would have taken 3 days to send now takes seconds. Email makes up the biggest part of traffic on the Internet. Documents, music, programs, greeting cards, mailing lists, catalogs, etc. can all be sent by email now as attachments.
With this power and flexibility also comes scammers, spammers, viruses, and spy programs. It is up to you to use email responsibly. Be careful about who you give your email address to, don't open attachments from people you don't know if you don't have to, don't click on links in email particularly for services like Ebay or PayPal or a bank, run current antivirus software on your computer, use spam or nanny software if you have young children using email.
The Internet as a whole works through a mutual understanding and agreement of millions of computers who relay Internet traffic including email. If any one node goes down, a section of the Internet may go down though there are numerous routes around such situations. It is also possible for email to be intercepted because of this so don't say anything in email you wouldn't say in public.
When you send an email, it is automatically addressed as having come from the address you entered in the settings of your program. Be sure this address is valid and kept up to date so recipients can reply to you. We see numerous replies from our support desk bounce due to invalid addresses. Be sure the address of the recipient is correct. Quite often a letter or period is left out. If the receiving server is down for any reason, email is usually cached and held until the server is back in service. This is also true for computers sending or relaying mail and why sometimes mail is received several days after it is sent.