Tony, pretty well anything in a computer can be edited/altered if you want to badly enough, if the person doing it is skilled enough it can be almost impossible for anyone other than an expert in forensic computing to tell what has been done. The only way to prove whether an email really was sent at the time it says it was would be to obtain the traffic logs of the relaying servers it purported to have gone through, unless you are a law enforcement agency you are unlikely to be able to do such a thing.