unable to authenticate to smtp.zoho.com

unable to authenticate to smtp.zoho.com

Hi, I have been using smtp.zoho.com to send emails from my python application.
A few weeks ago, it stopped working and I am receiving authentication errors in my app:

SMTPAuthenticationError: (535, 'Authentication Failed')

Here are the settings I am using, could you please investigate:

SENDER: admin@dressagenovascotia.ca

app.config['MAIL_SERVER'] = 'smtp.zoho.com'
app.config['MAIL_PORT'] = 465
app.config['MAIL_USE_TLS'] = False
app.config['MAIL_USE_SSL'] = True
app.config['MAIL_USERNAME'] = mail_username
app.config['MAIL_PASSWORD'] = mail_password
app.config['MAIL_DEBUG'] = False