In general you should not open messages, especially ones with attachments, from people/mail accounts you do not know or recognise. Also, be wary of messages that appear to be from someone you know but that are unexpected or unusual.
Click on the following links for more information about identifying and managing Spam email
Attachments - Is it safe to open email attachments even though I have anti-virus software installed?
How do I manage spam, junk email, safe senders and blocked senders?
Phishing emails - What are they?
Spam email - How can I avoid it?
Suspicious emails - What should I do?