This looks suspicious enough, it may be threat distribution or phishing attempt. It may look like coming from known user or customer but the message itself may be spoofed with forged email source address.
I believe most of the known email services providers have adressed several of these issue:
http://support.google.com/mail/bin/answer.py?hl=en&answer=50200
http://answers.yahoo.com/question/index?qid=20091209171845AAlUQsJ
...if you google for similar you probably find plenty of these.
As recommendation goes - good education of end users is required to be extreme vigiliant about emails coming with attachments or links - even when those seem to come from known adresses - it should be easy enough to confirm with the existing sender if he really have sent any messages.
+ ensure you are running dedicated email antivirus and antispyware protection - either directly on the mail server or from the level of mail clients (from SEP it can be Outlook, lotus notes or SMTP/POP3 protection component).