Microsoft is flagging one of our email addresses (quotes@xxxxx.com) and stating we are spamming from it (more than 30 emails per minute).  We only use this email send emails from DESK and BOOKS.  The ip address they believe it is being sent from is:

136.143.188.8 (Zoho IP address)

 We have checked Desk and Books but have no sort of email activity like this being sent out.  We check Microsoft portal for sent email for this account and do not have anything excessive. 

But Microsoft is adamant this email is attempting to send more than 30 per minute and are restricting our email address. This is causing all sort of email deliverability issues that we cannot resolve. This email address is being tagged as sending spam is beng restricted daily.

Would anyone have any suggestions on how to troubleshoot this?  We have Zoho, Microsoft and our IT firm all reviewing but nothing is standing out. Thank you any assistance.