SPAM automatically sent from my email account


New Email
Hi, I have just realised that last night an email was "automatically" sent from my email address to ALL my contacts (something about checking who has deleted you from the messenger). How has this happened? (it is in fact not the first time) and what can I do to avoid it happening again? (it is not only embarrassing but also inappropriate that many of the contacts in my list receive such email from me)
Thank you