I then removed an Exchange (ActiveSync) mail account. And the problem was gone. 30+ hours battery with no problems. I added the mail account back, and the problem reoccured. I testet this some more times and it was consistent each time. BTW I was not using push, just manual mail update.
The mail account that I am adding forces encryption on the phone, so it might be that this is the actual source of the battery drain. But it shows that the phone is useless using ActiveSync with encryption.
I was hoping that this was fixed in 2.3.4, but the problem was the same there as well. Until this is fixed (which I don't suspect will happen) I can not use the phone with my job email account because of this.