First make sure you have an outgoing mail server listed in settings;mail;youraccount
the server address will be relevant to the connection you are currently useing; eg, if you're using edge on o2, you need to use the o2 outgoing mail server, if you are using wifi , you need to use the current wifi suppliers outgoing mail server.
your outgoing mail server Host Name should be in the format
mail.yourserver.com:25
good luck!
(PS, scroll down in the account settings to find the outgoing mail server box)