When sending email from an Apple device the following error is received:
This is caused by the settings for the outgoing (SMTP) server being missing in the mailbox setup. When configuring a mailbox using the Mail app the username/password fields are marked as optional which can lead to confusion as this setting will be required. To fix this from the main screen of your device go to Settings > Accounts & Passwords and select the mailbox you need to update and go to Accounts > SMTP and update the two fields. Once saved this will then allow you to send using this device:
