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:
Who is Participating?
I wear a lot of hats...

"The solutions and answers provided on Experts Exchange have been extremely helpful to me over the last few years. I wear a lot of hats - Developer, Database Administrator, Help Desk, etc., so I know a lot of things but not a lot about one thing. Experts Exchange gives me answers from people who do know a lot about one thing, in a easy to use platform." -Todd S.

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.
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.
Mr.XAuthor Commented:
It's still trying to contact the old server which was decommissioned . Should I check something in new dns server ?
Big Business Goals? Which KPIs Will Help You

The most successful MSPs rely on metrics – known as key performance indicators (KPIs) – for making informed decisions that help their businesses thrive, rather than just survive. This eBook provides an overview of the most important KPIs used by top MSPs.

Dmitri FarafontovLinux Systems AdminCommented:
Check DNS settings on the client machine first.
Mr.XAuthor Commented:
It's Ponting to the new server
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.
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 ?
Dmitri FarafontovLinux Systems AdminCommented:
If you press View Certificate it will tell you what the problem is.
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/

Experts Exchange Solution brought to you by

Your issues matter to us.

Facing a tech roadblock? Get the help and guidance you need from experienced professionals who care. Ask your question anytime, anywhere, with no hassle.

Start your 7-day free trial
Mr.XAuthor Commented:
Tested and worked perfectly. It’s to do with service connection point.
It's more than this solution.Get answers and train to solve all your tech problems - anytime, anywhere.Try it for free Edge Out The Competitionfor your dream job with proven skills and certifications.Get started today Stand Outas the employee with proven skills.Start learning today for free Move Your Career Forwardwith certification training in the latest technologies.Start your trial today
Exchange

From novice to tech pro — start learning today.