Hi,
How are you determining which message you want to get the body for? There are numerous email-related tasks available, but the one that comes to mind is "Email - Get Single Message". It requires an email ID, so if you already have that, I'd suggest using this. If not, maybe try the "Email - Get Messages" task and add some criteria like a subject line, sender, etc to try to filter for the particular email you want. Once you have the email(s), there are several variables available to that task, and it wouldn't surprise me if the body was one of them. I've never actually used these tasks before though, so I'm not sure.