Server error: ‘550 5.7.1 Unable to relay’

Issue: User unable to send out email. Error message :Server error: ‘550 5.7.1 Unable to relay’

Solution:
1)Check email account settings.
2)Make sure smtp server is correct
3) In email account settings, more settings, Outgoing Server,  make sure there a tick for “My outgoing server (SMTP) requires authentication” and Use same settings as my incoming mail server is selected.