Based on the error message you are receiving, it seems that you are unable to connect your Microsoft 365 email account to Outlook on your new Android phone. Error 2400 is a generic error message that can occur due to various reasons. One possible cause could be that the version of Exchange Server that you're running uses TLS 1.1 or 1.0, which are deprecated and no longer supported by Outlook for iOS and Outlook for Android. Another possible cause could be that the user has an account at a third-party provider that uses the same email address and password that the user's Microsoft 365 account uses.
To resolve this issue, you can try the following steps:
- Confirm that ActiveSync is enabled for the user in the Exchange admin center.
- Confirm that the mobile device isn't blocked by an ActiveSync quarantine rule in the Exchange admin center.
- Create a new alias for the user in Microsoft 365 and use it to set up Outlook.
- Change the user name or password of the user's Microsoft 365 account or of the user's third-party account so that it is unique for the user's Microsoft 365 account.
If none of these steps resolve the issue, you may want to check if your Exchange account is already added to Outlook on another device, as mailbox synchronization might stop unexpectedly in such cases.
- The connection to your mail server timed out error in Outlook for iOS or Outlook for Android
- Connection to Microsoft Exchange is unavailable error when a Microsoft 365 user tries to set up Outlook
- Microsoft 365: Outlook and mobile device connectivity troubleshooting resources
- A mobile device can't connect to Exchange Online by using Exchange ActiveSync