Mail not sending on iPhone? Check SMTP settings

Today I fixed a coworker's iPhone which could not send emails from both of his email accounts on his phone.  I discovered that the problem stemmed from the SMTP settings configuration, which did not enable authentication by default.  To correct the problem you will need to enable authentication under the SMTP settings pane; most people will be fine choosing password authentication.

 

The final pane is where you need to be in order to change the SMTP settings.  The server port will vary depending on your service provider.  Newer implementations will use port 587, some servers will use port 25, and yet others will use port 80.  Not all mail services will support SSL, but I highly recommend using it if it works with you server.
One last thing to note: In my example, I use gmail.  Setting up a gmail account using this method should not be necessary, as iOS implements gmail correctly and automatically configures the settings correctly.