Were you ever at the same email provider as Ex-Sell?
I can see that their MX records are as follows:
ex-sell.com MX preference = 10, mail exchanger = mx1.emailsrvr.com
ex-sell.com MX preference = 20, mail exchanger = mx2.emailsrvr.com
Is there a chance that your domain used to be hosted on these same servers?
If so, there is a chance that emailsrvr.com think that they are still authorative for the rippleeffectmarketing.ca. So when ex-sell send an email via their own servers, the server thinks that your domain is hosted there and tries to deliver it locally (therefore not sending it on to Office 365)