A bit of google-fu has hopefully found some info you can use.
Firstly, if your shield does have the SIM900D chipset, then email appears to be a somewhat straightforward set of AT commands outlined here in the SIM900 Email AT Command Manual
(PDF). In the introduction paragraph it confirms the SIM900D conforms to this operation. From a quick scan through, so long as you know your SMTP from your POP3 this should be helpful.
For MMS I think the answer is probably in sending an SMS in PDU mode (rather than text). There's an explanation of what's possible here
(I didn't read all 17 parts, but it looks to be pretty thorough). And it appears many of the sites that talk about operation of the SIM900 based shields (like this one
) can help you get an SMS started in PDU mode.
I've not done this, just hate seeing a 0-reply post...so hope something here can be of help to get you on the way, cheers ! Geoff