I have a client that sends and receives email in Zoho CRM using her Outlook (Office365) email. I set up the account to use the Office365 Mail and SMTP servers, and for a couple of years it has worked fine. Recently, many of the emails she sends are bouncing with the error message "Remote Server returned '550 5.7.520 Message blocked because it contains content identified as spam. AS(4810)'".

The email is not very spammy, and it has worked fine in the past, so I suspect there are enough authentication issues that it is now triggering the SPAM filter. However because it is an Outlook address, not a company domain URL, I can't add an SPF or DKIM record.

Interestingly, auto-response emails (which I believe are sent by the Zoho servers) are not bouncing. 

Any ideas on how to improve deliverability? Would it be better to use the Zoho mail servers rather than the Outlook servers?