suddenly unable to send photos via email from my iPhone 4

I sent a couple of photos from my camera role to a friend last week but she says she didnt received them - I checked my sent folder and they are not showing, I have tried several times since then and its the same thing...i open the photo, click the little forward arrow sign and opt to send via mail, add adress, chose size (always medium) and then send - the phone makes the wooshing sound to say its been sent but then I check and the email & photo has not arrived and its not showing in sent folder either???  

I was sending photos this way on lots of occasions up until a week or so ago with no problems at all.    can anyone offer any ideas on what this could be?

Thank you so much :-)