SMTP error: could not authenticate

FYI: I host a few clients on a reseller account. I don't do PHP, hence this message. The

url/website in question: https://www.photohippo.co.uk/management/film.php?action=home

In the client's own words... "Our management system stopped sending emails over the last couple of days. The "Send 'Received' Email", "Send 'Payment' Email" and "Send 'Completed' Email" buttons don't send when clicked."

SMTP Error: "Message could not be sent. Mailer Error: SMTP Error: Could not authenticate."

I would need the admin password and the DB Password amended in the config file and anywhere else that may be appropriate. And I need the SMTP error on three forms in the management software fixed. I can give access details to cpanel, and the management software as required. Full details given if you can demonstrate you know your way around PHP and SMTP stuff. Thanks.