When migrating Exchange mailboxes to Office 365, KEMP recommends having the migration services bypass the LoadMaster. This requires changing your DNS record to point away from the Virtual Service IP address and go directly to the proxy server itself.
If you choose to have the migration go through the LoadMaster, KEMP recommends using a separate Virtual Service. Configure this Virtual Service for SSL "pass through" which means no SSL termination is done at the LoadMaster. The Mailbox Replication Service (MRS) proxy service expects the traffic to be signed/encrypted, which means it cannot be offloaded. This service expects traffic to be encrypted throughout the entire process.
For further information, refer to the following Microsoft TechNet article: https://technet.microsoft.com/en-us/library/dn635115(v=exchg.150).aspx
KEMP recommends using a separate Virtual Service for the MRS proxy with SSL Acceleration disabled. Due to the fact that many customers are leveraging features such as SubVSs, ESP, WAF and content switching, we do not suggest disabling SSL Acceleration on the production Exchange service(s).