Email reception issue after Office 365 tenant migration.
We have hit the 25 users limit of our Office 365 Small Business Premium subscription. For some reasons beyond the scope of this question, we could not use the subscription migration wizard. We have thus purchased another subscription (Midsize), recreated the mailboxes on the new tenant, transfered the emails and attached our DNS name to the new subscription. Détails of the procedure are here: https://community.bittitan.com/kb/Pages/How%20do%20I%20migrate%20from%20Office%20365%20to%20Office%20365%20while%20keeping%20the%20same%20domain%20name.aspx
We can send emails and receive email, except for one situation: when someone whose email is hosted on Exchange 365, send us a mail, we do not receive it. Delivery reports do indicate that the email has been delivered. Moreover, mail flow traces do show that the email has been delivered. However, it is absent from the destination mailbox (and its Junk folder). We've also checked the mailbox on the old tenant (we still have access to this using our 'onmicrosoft.com' credentials), there is nothing there.
Any idea would be appreciated.