I can receive & forward mail on my iPad but cannot send it.

Settings - Mail, Contacts,Calendars - <telco account name> - delete it from iPad (you are not deleting the email account with the provider just removing it from the iPad)

Try a forced restart - After shutting down all open apps using the app switcher - invoked by quick double press of the home button and swiping up on each app till it flies off the screen.

Hold the sleep/wake and home buttons down together till you see  logo - then release and allow normal boot up

Re add the account back to the iPad mail app

See if issue resolves

Sending and fowarding use the same servers and settings. If you can do one , you can do the other.

How do you know you can forward? What error message do you get when sending?