Upgrading your organization from current versions to Exchange Server SE

Mr_Tomasz, EMS E3 is not the same as Microsoft 365 E3, and it does not provide the same extended use rights. See Microsoft Product Terms for full details. But, to get EMS E3, you do need to have a qualifying licensing, like a Core or Enterprise CAL Suite which would give you the required user/device CALs for Exchange Server.  However, EMS E3 does not provide free Server licenses like ME3 does.  So, you would still need to get Server licenses (that include SA or come via a subscription with extended use rights).

As for usage, yes in all scenarios the product is fully operational and supports external email.  All we're saying is that if you go the ME3/ME5 licensing route for your users, then all Exchange Server users and devices in your org need the same license.