We are running into some issues with migrating from Office365 to Zoho Mail.

Here is the setup

1. a@culytics.com was hosted on Office365 and is being moved to Zoho Mail.
2. b@culytics.com is hosted on Office365 and will stay there for some time.
3. c@culytics.com is hosted on Zoho Mail
4.  

Now the issue is

1. b@culytics.com is not able to send email to a@culytics.com. We get email not delivered error - error code 5.1.10 in Office 365,
2. c@culytics.com is able to send emails to a@culytics.com.
3. I am also able to send emails to a@culytics.com from Gmail.
4. a@culytics.com is able to send emails to b@culytics.com and c@culytics.com

What might be the issue and how to fix it?

 Thanks.