If the SMTP server requires authentication, check that you have defined the proper authentication method for your SMTP server in Accounts > (email account) > Outgoing Mail Server > Authentication. Select either Plain password or Encrypted password, depending on which type of authentication is required by the SMTP server.
Check also that the other Outgoing Mail Server settings are properly defined. In addition to the authentication method, verify the settings below with your email admin:
- Server address
- Secure connection (None / SSL / TLS)
Check also that a correct password has been defined for the email account.