Migration from Exchange 2010 to 2016: internal DNS question

I'm trying to add an Exchange 2016 server to my infrastructure and to migrate everything there from an Exchange 2010 server. I'm following this tutorial: https://practical365.com/exchange-server/migrating-exchange-server-2016/

But I get stuck at the Client Access Name cutover since my DNS configuration seems to be different from the one in the tutorial.

So we've got an internal DNS server for our internal network, and an external DNS server (outside of our AD, of course) for our public addresses. But in the tutorial it seems like they're using the same domain name for internal and external purposes?

Anyway, my Outlook connects via RPC/TCP to the Exchange server using its internal name (serv-exchange.whatever.local). But I don't know where this has been configured? Can I force Outlook to use the external name (mail.mydomain.com)?

That would make the Client Access Name switch easier ... I'd just need to change the NAT on the firewall for mail.mydomain.com ... otherwise I would have to change the DNS config of the current Exchange server which would create all kinds of problems, I guess?

I've tested accessing the new Exchange 2016 server on a client with a modified HOSTS file (with serv-exchange.whatever.local pointing to the new Exchange 2016 server) and that works fine! But in order to do that on the level of the internal DNS I would have to point serv-exchange.whatever.local to the IP address of new Exchange 2016 server but then the connection to the current Exchange 2010 server (where all the mailboxes still reside) would get lost ... Speaking of lost, I am quite lost right now! :D Any help would be appreciated!
XeronimoAsked:
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Jeff GloverSr. Systems AdministratorCommented:
Teh reason for using the same name internally and externally has more to do with Certificates here. You can no longer put a .local or other private domain in a public certifcate. Exchange needs one certificate for IIS access (this covers most normal things. SMTP, POP, IMAP and UM are different). Your best bet here would be to put in Split Brain DNS. Where you make a copy of your external DNS and host it internally. Any internal sites, you would simply change the IP address in the A record. So say your IIS on Exchange (your external CAS name) is mail.company.com and you have an external DNS provider for that (like Godaddy or similar) and your internal Domain is company.local. You would create a zone on your internal DNS for Company.com and duplicate records there except where they point to your firewall mappings. You would change those records to point to the internal servers.
  So, someone external to the network going to mail.company.com would be directed to the IP address on your firewall that is mapped to Exchange and someone internal to your network going to the same address would go to your exchange server internal address (or load balancer VIP if you are using one)
XeronimoAuthor Commented:
Yes, I get that ... but right now my Outlook 2016 is connecting to my Exchange Server (2010) via RCP/TCP using the .local name ... how could I change that?

Even if I recreate my domain.com zone on my local DNS server, Outlook would still try to connect using the .local name, no? How could I tell my Outlook to LOCALLY connect to the new Exchange (2016) Server via https://mail.domain.com?

Thanks!
XeronimoAuthor Commented:
Although, wait, that's to be expected since the mailbox is still on Exchange 2010 ... I've tested it with one on Exchange 16 and there Outlook uses https and MAPI.
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