The internal/external URL on sbs 2011 is aready mail.domain.com. Is it safe to assume that the exch 2016 internal/external urls shouldn't match? Also, at what point do I change the incoming smtp and ssl ports to point to the new server? i had 600GB of mail to migrate so thus coexistence needs to be intact for a while.
Are you sure the URL's for Exchange 2010 on your SBS 2011 are 'mail.domain.com'? Normally those are remote.domain.com when you run the wizard. You can have both servers have the same internal and external URL's during the migration because they are really only 'listening' names.
I would change incoming SMTP to the new server as all batches are complete. Are you going to use ARRConfig as I suggest in the guide? If that is the case you need to route traffic for SMTP to the internal IP address of your Exchange Server and traffic on port 443 should go to the new 2016 server. ARR takes care of routing traffic to OWA and such.
I am 100% certain that’s what the internal and external urls for the SBS 2011 server are. During the intial setup of SBS (and even after), one can specifics a name other than remote.domain.com. I suspect it was set up this way because the mail server prior was configured for mail.domain.com. As that what users are use to, it needed (and still needs) to carry over as everyone's smart phones are configured to use that url also.
No problem at all. Just use the certificate from the SBS also on the new Exchange Server. It can not be a self signed certificate. As soon as you change DNS entries for your new Exchange Server locally and install ARR those external clients will connect to your new server.
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