Some ISPs (Internet Service Providers) will block sending mail on the default port 25. Follow the Steps Below to take you through changing your port to 587 which will allow mail to be sent.
1. In the Mail app, select the settings icon.
2. Underneath Settings, tap Accounts.
3. Underneath Accounts, tap The account you want to edit
4. Click “Change Mailbox and Sync Settings”
5. Click “Advanced Mailbox Settings”
6. Add :587 on the end of the outgoing server and click Done
7. Click Save.