Migration from Exchange 2013 with Sophos Gateway to Exchange Online with Sophos Mailflow

Question

We are currently migrating from Exchange 2013 with Sophos Gateway to Exchange Online with Sophos Mailflow but unclear how to proceed. I've requested support a number of times but they keep pointing to a document that that outlines the move M365 Gateway to M365 Mailflow which isn't what I'm doing. 
 I really need a step by step guide on how to go about this an timing. I've completed a 90% data migration of all mailboxes so would the Sophos move take place now or after a complete migration, where would this leave the on-site server if moved now? 
 The first stage of the setup asks the question am I copying the M365 domain or setting up domain manually, I'm assuming manually as I assume exiting means I'm already using the gateway product for M365? Any pointer would be helpful.

emmosophos · Answer

Hello Simon, 
 Thank you for contacting the Sophos Community. 
 I would recommend you to reach out to your Sales Engineer or Professional Services so they can assist you with your migration and new configuration. 
 This type of scenario is outside of the scope of Support. 
 It does seem you&rsquo;ll fall between a hybrid environment, where some users will remain on Exchange and others on M365, I would move every user to M365, and once that is done, start the configuration on Central for the Mail Flow, and yes you would be setting it manually since you are already using it previously in Central. 
 But again as I mentioned I highly recommend, you reach out to your Sales Engineer or Profesional Services to assist you with this. 
 Regards,