brightmail fake mail

Hi,

We have a problem about mail security,we saw an issue about our mail clients,

We have a user who name is ayse mehmet.She has an email address which is ayse.mehmet@seabank.com.

However,she is receiving  some mails from ayse.mehmet@seabank.com,we wonder that how it is possible that situation?because she says "I didn't send these mails".These mails are coming from outside,not inside.

We wonder that is it a virus?how can we prevent these emails?Can we use brightmail for this problem?

thank you,

Best regards.