Why you can still receive email in your previous POP3 mail service provider after migration to Office365

a) Some of the mail servers may have cached their dns request long than expected or may have hardcoded the dns resolution (reason for speed). 
b) The sender’s mail server is in the same mail service environment as your previous mail service provider.
To resolve this, you will need
a) to inform the mail administrator of the sender about your migration. Ask the administrator to remove any hardcoded DNS resolution if any..
b) to remove all mail accounts in the previous mail server and send email to terminate the hosting services with the previous email services provider.