Sometimes Yahoo blocked an email from mobile devices with the error:
Sending the message content to the server failed
If this happens to you, try removing any links or other content that could be confusing the Yahoo spam filtering systems and then try sending the email again.
Does it work better if you try using the Yahoo Mail App instead of the default mail program that comes with the iPhone?
If you are using the default mail app you can also try creating the Yahoo account preferences manually and use your ISP's smtp relays if it's a consistent problem.