If you are having issues sending email on an iPhone or iPad, please check your SMTP server setings and verify you
are using Port 587 with SSL Enabled and your username and password entered in their respective fields as shown below.

You can access these settings by clicing on the silver Settings icon on the home screen

From here you either go into accounts and passwords and select the account, or go into mail and select accounts.
Different versions of IOS have the settings in different locations.