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I recommend that you contact your email provider and check if their SMTP-server supports encryption and another port then 25. If so, edit the email account and change the settings for outgoing emails to use encryption and the port specified by your email provider.
If you can't use encryption with your current email provider, contact your mobile operator and ask for their outgoing email server details and enter those instead. You may however not be able to send emails using WiFi in this case.
This is not a setting in your phone. What is happening is that either your email provider is not allowing you to use their SMTP-server (used for sending emails) when you are connecting from your mobile operator or that your mobile operator is not allowing you to use an SMTP-server outside their network.
The general solution for this is to either use the mobile operators SMTP-server for sending emails when using mobile data. In case you can connect to your standard email providers SMTP-server using SSL on another port then 25 that will usually work as well.
In case you have a Gmail account you can try sending email using their SMTP-server:
Outgoing server: smtp.gmail.com
Encryption: SSL/TLS
Port: 465
Username: your gmail address
Password: your password