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.

Resolution :

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)#policy-map global_policy
pix(config-pmap)#class inspection_default
pix(config-pmap-c)#no inspect esmtp
pix(config-pmap-c)#exit
pix(config-pmap)#exit