If you try to send an email from your iPhone and get an error message that says "A copy has been placed in your outbox. The recipient was rejected by the server because it does not allow relaying," chances are you need to check your outgoing mail server settings. Please follow the below steps:
1) Go to Settings --> Mail> Contacts> Calendar and tap on your email account.
2) Tap on "Account," then go down to the "Outgoing Mail Server" section and tap on "SMTP."
3) Tap on your Primary Server.
4) Even though it says "Optional," fill out both username and password under Outgoing Mail Server.
5) Tap "Done" in the top right to save your changes.