How can I safely open a suspicious email?

Suppose I receive an email that I highly suspect is some form of spam but I'm not 100% sure. Suppose also that I'm using form of webmail (like Gmail) with good spam filters, but this message made it through safely.

Obviously I should not open any attachments, but is this email otherwise safe to open? If not, is there a simple way to safely open it?

If you open the email in a web client (ie, online at gmail.com or mail.yahoo.com, etc), you're generally very unlikely to experience any problems. If this email contained a script virus (very rare nowadays) it would generally require being opened in an email client locally installed on the computer in order to gain sufficient access to actually infect your computer.

Viruses, because of the popularity of web clients for email, have pretty much stopped sending themselves as emails in the last few years.

Spam is still a problem, and many viruses create spambots and enslave their infected computers as spam relays. But you're not going to catch a virus from the average spam message.

If you are using a local email client, don't open suspicious emails unless you've got the email client running inside a virtualized system that you can scrub easily with a reset.

If you want to be really safe, you can install a virtual machine and an operating system on it so that you can open the files there, this way it doesn't affect your OS if something goes wrong.