Normally all you need to do is set up the email app for fully secure authenticated sending of email and it just works.
What mobile network are you sending from? We have seen a recent report from a customer using the 3 network saying they had to use the 3 network SMTP server authenticated with their 3 username and password to be able to send emails.
The pre-condition is that the mobile data connection is good and that you can get an Internet connection. i.e. normal websites are viewable on your mobile. Technically that the Access Point Name is set up to get your mobile service an Internet connection.
Then that the email app is allowed to use mobile data and has not been restricted to use wi-fi only.
And finally that the email app settings are for fully authenticated secure sending. The email settings are Email settings - IMAP or POP3.
If you still have difficulty the easiest way to see what you've got set up is to take screenshots of the incoming and outgoing server settings, image edit to obscure the first part of your email address and then post here using the photos icon above this Reply area.