We have two Exchange 2010 servers (SP3 RU26) but decided to add a Exchange 2016 (now CU12) server as our "hybrid server", in other words, the server that will be retained as the still necessary on-premises server (Exchange 2010 will soon be EOL). I ran the HCW (hybrid configuration wizard) on the Exchange 2016 server and noticed that the Exchange 2010 servers were not selected or apparently selectable as source servers for the new " Outbound to Office 365" send connector.
We migrated some test mailboxes to Exchange Online and those mailboxes can send test messages to recipients whose mailbox is located on the Exchange 2010 servers. However, we cannot send test messages to the mailboxes migrated to Exchange Online. I created a test mailbox on the Exchange 2016 on-premises "hybrid server" and I can send to Exchange Online mailboxes from THAT server. But still not from mailboxes on the Exchange 2010 servers.
Should the Exchange 2010 servers be added to the " Outbound to Office 365" send connector as " source servers" ?
I understand many things could be causing this problem (firewall rules and the fact that the 1 to 1 NAT only includes, by definition, the Exchange 2016 server) but I want to rule out the absence of the Exchange 2010 servers as source servers in the "Outbound to Office 365" send connector.