Yes, it absolutely needs investigation rather than being either believed or disbelieved straight off.
The email names a supposed witness to the alleged sexual assault, which is somewhere for the police to start.
On thing that's really struck me regarding the TV presenter, is that the investigation is being carried out by their employer, the BBC.
The police have looked at the presenter allegations twice now and said they haven't seen evidence of criminality and are no longer investigating.
So what is coming out relies heavily on the BBC, which has access to his work phone. If this person had been working for a different employer or been self-employed, who would be doing this investigation? Possibly no one with adequate access.
Eg I can't see The Evening Standard carrying out an investigation into this, especially with daily public statements on progress, even though Osborne was working for them at the time of the alleged behaviour.
People have been talking a lot on other threads about how powerful men get away with sexual misbehaviour. If Osborne's behaviour were to turn out to be sleazy or abusive, but not criminal, then unless alleged victims were willing to put themselves out there in public, it might never get beyond the point of being rumour and gossip.
Osborne may, of course, be pure as the driven snow and every word of the email might be untrue, even the claim he was having an affair with Thea.
Without a full investigation, we can't know.