Messages still go to the old mail server after email migration.

My client migrated email from normal cpanel hosting to Outlook Office 365 service. Migration was successful, DNS changed.

The client normally receives emails to the new Outlook service. But somehow the Contact Form messages sent to the client’s email address still arrive at the old mail server (only these messages).

This is surprising because the DNS was changed already 2 weeks ago, so it’s practically impossible.

Do you know what could be the cause?