I am using Verizon Samsung Fascinate running the M5 AOPK ROM and using Handcent messaging app.
1. In stock messaging app, only check the option for auto-retrieve for MMS messages.
a. after you confirm the initial steps work then feel free to play with whatever else you want such as delivery reports and roaming auto-retrieve etc.
2. Go to your messaging app. I'm using Handcent so my options will be different but the essence is the same unless of course you use Handcent. Go to settings and application settings. Select Default Messaging application and choose SMS.
3. Go to Send Message Settings and deselect Auto Convert to MMS. Also select Maximum MMS Size and choose default of (1M).
4. Go to Receive Settings and deselect Auto Retrieve (Under MMS, this is controlled by stock messaging app).
That's it! Again, feel free to play around with anything above but just know the ROM is NOT at fault. Hope this helps.