Yes. If you've sent it once, it's better to send complete with the header information, and date and time it was sent (just forward the original e-mail to them, that will contain everything).
Lots of people are prickly, and the above approach may insult them. There are many times when it's better to resend it without forwarding the whole e-mail for the sake of civility and/or saving yourself a headache.
Some rules of thumb:
- Do forward it if it is time-sensitive information (so you don't get blamed for delays)
- Do forward it it's a coworker who you're training and shouldn't lose the e-mail in the first place
- Don't forward it if it's an estimate to a client and their boss was CC'd (making someone with purchasing power look bad to his boss is bad form)
- Don't forward it if it's your boss and other coworkers are CC'd (your boss doesn't want to look bad in front of the other coworkers)
These are all just rules of thumb. Use good judgment. Generally speaking, the idea is that you want to protect yourself first, but not protect yourself from marginal risk at the expense of making the other person look bad, especially in front of their boss/coworkers.