The problem is it's difficult to distinguish donating money in order to get a politician to support your legislation from donating money because a politician supports your legislation.
You really need to see what's in the email record and, as you said, check out the dates of contributions and any change of support and see if there's a connection or a pattern.
An honest politician could hear out a lobbyist, agree that his legislation is worthwhile, then receive a donation since he's now a supporter of that legislation. If I was in elected office I'd take care to make very clear to my constituents the reasoning behind why I was voting a certain way, particularly with any changes in positions.
of course. everyone who donates to a politician, even the $50 I send, is sent because we want them to vote a certain way on various issues.