Your email service comes with a few size limitations. Attachments, particularly ones with images and/or lots of data, can get really big, really fast. Big attachments are a problem for a couple of reasons. Some email accounts restrict the size of individual messages. Messages that are too big don't get sent, which might be what you're experiencing. Alternately, if your email account is full (has reached its size limit), you can't send (or receive) ANY messages until some messages are deleted.