Some of the users are not able to connect to their mailbox through active sync

Hi John,

Try to troubleshoot with https://testconnectivity.microsoft.com/ select Office 365 and ActiveSync.

It will gives you the results that what is happening.

One thing that comes to mind is MFA, but there can be a variety of factors. Do some basic troubleshooting or at least provide all the relevant information so we can give you some ideas. Or open a support case to have this properly investigated.

Are you sure ActiveSync protocols are enabled?

Get-CASMailbox <user> | select ActiveSyncEnabled

No idea. And as most of the classic troubleshooting techniques (https://www.slideshare.net/himickey/exchange-active-sync-troubleshooting) don't work with Office 365 because you don't have access to log files, you should file a support incident.