- Go to Settings -> Accounts and Settings
- Tap on your email account
- Tap on "Account", then navigate to "Outgoing Mail Server" and tap on SMTP.
- Tap on your primary server
- Even though it says "Optional", fill in both the username and password under Outgoing Mail Server.
- Change the Server Port to 465 and ensure that "Use SSL" is on.
- Tap "Done" in the top right to save the changes.
You should now be able to send emails without receiving the above error.