Hello everyone,

I have on-premise Exchange 2010 which is a local mail server, only local users use to exchange emails locally does not have any MX records on public DNS. and other users have mailboxes on office365 which configured on outlook clients as pop3 email accounts so every user of those have 2 accounts on outlook and of course the doesn't receive any emails from on premise users.

I decided to merge the office365 accounts so i started with the ADD client syncing them with Azure AD and merged smoothly and the passwords have been changed to the local accounts passwords.

no i am trying to use HCW to merge mailboxes and i have 2 concerns.

1- Do i have to buy a public SSL cert to use it between on-premises Exchange server and office365 or the local one will do the job?

 2- As the on-premises exchange 2010 does not send email outside LAN, will the office365 users be able to receive messages from on-premises users?