Suspicious emails are being sent from my email account

Suspicious emails are being sent from my email account

Suspicious emails are being sent from my email account.  This morning I received 4 bounce back messages that an email was undeliverable.  All 4 email bounce back messages are not recognized and all 4 "to" email addresses are not recognized.  It would appear my account is sending emails that aren't coming from me.  How can this me fixed?