how is someone able to use my email

[The Dad Fisherman]

Not really to much you can do. Its probably getting your address out of someone elses mailbox. Make sure you always put in a real ly good descriptive subject line to exactly what this e-mail is pertaining to or Come up with a standardized form for customer contacts. People need to realize that the problem isn't just a switch you can flip. They need to be Pro-active and look at e-mails when they get them and:

A: Make sure you know the Person who Sent it
B: Look at the subject Line and determine if Said person would send me an E-Mail w/ A blank or simple Subject Line
C: Am I expecting anything from this person

If it doesn't match up to one of the criteria either delete it or send a message to that person to clarify if they sent it or not.

Keeping your Anti-virus up to date helps, get the updates on a nightly basis, But there are dozens of new viruses that come out every day and one will slip through.