Ok can anyone help me out with an At&t samsung Captivate (Smart phone) ?

Ok It won't let me send or view MMS ( pictures) it has been going on for a while now and every time I click on download when some one sends me some thing it always says " Currently unable to download. Please try again later." ??!!? Can anyone help me because I've looked online and stuff people have the same issues as me but they never give any help.. :/