Constant SMTP errors

I send our response notifications via Office 365 Exchange Online SMTP, and this has started failing constantly today.

Am I able to access some logs to tell me why the messages are getting rejected?

SMTP is configured as per the Microsoft Office 365 SMTP setup, so not sure what else I need to be doing. It's been working well up until now.

If you are using your own SMTP then, try to white-list our Jotmail IP addresses within your email server (or ask for it to your email service provider). Read this article for more details: http://www.jotform.com/help/145-Whitelisting-JotMails-IP-Addresses 

Hope this helps