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 366 which will allow mail to be sent.

1. In the Mail app, select the settings icon.

2. Tap “Manage accounts“.

3. Under “Manage accounts”, tap the account you want to edit.

4. Click “Change Mailbox Sync Settings

5. Click on “Advanced Mailbox Settings

6. Add “:366” on the end of the outgoing server and click “Done

7. Click “Save” to confirm and complete setup.