AD domain (with exchange server) and office 365 domain same name

Hi, i have a internal AD domain (Server 2012) with exchange server 2013 say abc.com. Only with 15 users

My objective
i am planning to move all emails to office 365.
i don't plan to purchase public  SSL certificate, setup federation and exchange hybrid mode. i want to decommission internal exchange server

External network user currently able to connect to abc,com office 365 using outlook 2013 but the internal users still not able to do it.
i believe this has to do with the internal dns. i tried adding the autodiscovery.abc.com to my internal dns but still not working,  I believe there are more to do..

Kindly advise what is the simplest way to resolve this issue without additional cost
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dnackAsked:
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FOXActive Directory/Exchange EngineerCommented:
In the link below scroll down to post migration tasks and verify if you have done all.

https://support.office.com/en-us/article/Perform-a-cutover-migration-of-email-to-Office-365-9496e93c-1e59-41a8-9bb3-6e8df0cd81b4

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dnackAuthor Commented:
yes. I only need to run the following command to get things working as  I have previously amended my DNs

Set-ClientAccessServer –Identity “CASxx” –AutoDiscoverServiceInternalUri $NULL

where CASxx is my one and only server internal hostname.

By the way, how do I undo this command?
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