Hello, staged small office 12 users mailbox from on-premise exchange 2007 to office 365. Change MX record and autodiscover CNAME to 365 addresses. I have not removed any on-premise mailboxes yet and exchange is still up. From within the network, autodiscover does not work however manual outlook settings conenects them to their 365 mailboxes. Only problem is when they first open outlook, they get a security alert about their old mail server (mail.oldserver.com) name does not exist in outlook.com certificate. What can I do to eliminate this popup? I don't won't to delete mailboxes just yet because need to archive old emails locally. Any suggestions appreciated.
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