I, too, discovered my H's affair on email. There was a nice little timeline laid out there, and it was clear that he was the pursuer. Once she said "yes," it took him less than 48 hours to end our marriage.
The interesting thing is that he considered my reading his emails to be crossing a much bigger boundary than his leaving his family for another woman. And so did his family and many friends. For the life of me, I'm not sure I'll ever understand that. And it was his "reason" for moving out and changing his mind from separation to divorce--within 2 days of the bomb, and on my birthday.
M60 H52 D20 M14 yrs OW-old gf from 1986 bomb-5/18/08 H filed for D-9/10/08 D final 4/24/09 xH remarried (not OW) 2012