Sometimes you will face the below error message in horde (Not in Squirellmail and RoundCube) while sending the email.

” There was an error sending your message: Failed to add recipient: [SMTP: Invalid response code received from server (code: 550, response: Authentication failed)] ”

Solution :
Login to the horde.Click “option” button at the top and fillup the Personal Information of that email account.Now logout and login again and try to send the emails.