Just tried it on my iPhone 4 with iOS5, AT&T, and it worked OK - so it's not an iOS5 issue apparently.
I don't see an outgoing email in my Gmail account, so I presume this is being sent as a "Text" from AT&T to an email address destination. This implies carrier support required. Did you change carriers with your 4S?
Can you create a new text message to an email recipient and have that go through successfully?
Edit: Return address on the email was "XXXXXXXXXX@mms.att.net" so definitely sent through AT&T.
If forwarding it to an email address that's attached to iMessage it shows up as trying to send it via iMessage -- it will send it but it never shows up in email, probably because it was sending via iMessage.
If I forward to another email address not associated with iMessage, it shows up as sending via MMS -- and it does go through to that email address as an MMS.
Obviously this isn't very intuitive, but it does help explain what you're seeing. It would be nice if it figured out you were forwarding it to one of your email addresses and used email to send.