server alert error in outlook after migrating to office 365 from exchange.

hi guys,
recently my senior tech did a migration from exchange server to office 365.
everything works fine. but still for some user, in outlook they get the security alert message from the old server regarding certificate validation.
if users just press yes. everything works fine.  but everytime when they close and open outlook this message pops up.  could anyone help with this? is this something related to dns ? still pointing to old server ?

attached a pic of the error.
errorerror
Mr.XAsked:
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Dmitri FarafontovLinux Systems AdminCommented:
Press on the view certificate option. What do you see there? You get a certificate mismatch and thats the root cause of this is error.
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Dmitri FarafontovLinux Systems AdminCommented:
This expert suggested creating a Gigs project.
If the problem affects only one user a quick session in Gigs can solve it the fastest.
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Mr.XAuthor Commented:
It's still trying to contact the old server which was decommissioned . Should I check something in new dns server ?
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Dmitri FarafontovLinux Systems AdminCommented:
Check DNS settings on the client machine first.
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Mr.XAuthor Commented:
It's Ponting to the new server
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Dmitri FarafontovLinux Systems AdminCommented:
Do an nslookup on the clients command prompt. What server does it go to? Or start Outlook with /rpcdiags mode to see where it's connecting.
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Mr.XAuthor Commented:
Hi I did nslookup, it's showing the correct DNs server address. And one thing I forgot to mention before is. Old exchange server and new server has same ipaddress.

And did rpcdiag, it's showing the server name as outlook.office365.com . I don't find any problem here.

One more thing is found is.
When I did nslookup, set a=mx, it shows the server as DNs server address and then in non- authoritative answer: shows outlook.office365.com
Does it mean, it first tries to contact the server ip and then goes to offcie365 ?
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Dmitri FarafontovLinux Systems AdminCommented:
If you press View Certificate it will tell you what the problem is.
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Mr.XAuthor Commented:
I think I have fixed this issue. Waiting for few days from clients, whether they are coming back with same issue.

I disabled service connection point in registry in one computer, seemed it fixed the issue.  So made a gpo to apply to all computers.

This is the link which helped me. Hope it's fixed permanently.

https://www.itjon.com/outlook-still-trying-to-connect-to-old-exchange-server-after-moving-to-office-365/
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Mr.XAuthor Commented:
Tested and worked perfectly. It’s to do with service connection point.
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