Error migrating user from office 365 to on-premises
After migrating a user from on-premises exchange to O365 and try to move it back from O365 to On-premises the user will not move and you will see a message similar to the one below.
To resolve this issue, you will have to disable the ESMTP Inspection rule on your Cisco firewall.
The commands to disable ESMTP inspection are:
pix(config-pmap-c)#no inspect esmtp