hi OP
I'm so sorry this has happened to you.
this must all be very raw right now and you'll also be in shock and probably struggle to organise any of your thoughts for a while- this is normal under such horrible circumstances. I'm glad you posted, lots of us on here have experienced breakups and the trauma that inevitably ensues to some degree or another.
I would say to trust your gut as I believe you need to until/if evidence is produced that his email was hacked (unlikely but let's wait and see).
If he is adamant that this is true, just demand to see his phone straightaway (no notice given) and check for photos etc on it - no doubt he took several photos to check his dick looked as 'attractive/big' as possible before sending them to her. I
bet he's saved her pics too so he can have a wank over them whenever he wants - men in these circumstances are very predictable.
You can also check bank statements for anything that you don't recognise - these sites/contact groups mostly do have fees and they'll probably be under some innocuous reference like 'XYZ Holdings Ltd' - not obvious like 'Cheating Twats R Us'.
If you do this (and you don't HAVE to - just if you want to), at least you'll know if your gut was right.
As for the divorce thing - I'd feel the same - 100% marriage would be over ......but you don't have to tell him or DD until you're good and ready and have got legal advice etc. Keep this to yourself and go through the process of ducks in a row before saying anything - it'll just pour fuel onto the issues already happening.
If it helps, when it happened to me I just kept saying to myself that whatever I did or said was an example I was setting to my DC and it helped me to choose better behaviour (well for some of the time!).
Wishing you lots of courage, hugs and remember that this is a very emotional journey you're on and you'll have extreme feelings (good and bad), so please take the time to care for yourself - time out when you need it, connect with family and friends IRL and take hundreds of deep breaths.
You WILL come through this, I promise xx