When an Exchange Online user has access to a shared mailbox through full access permissions, he can reply on behalf of that address. The problem is that, by default, emails sent as the shared mailbox are saved in the user’s main mailbox instead of the delegated mailbox. To fix that you have 2 options:

Option 1: Use Exchange Online Powershell

For emails sent as the shared mailbox (Full Access)

Set-Mailbox <shared mailbox> -MessageCopyForSentAsEnabled $true

For emails sent on behalf of the mailbox (Send On Behalf)

Set-Mailbox <shared mailbox> -MessageCopyForSendOnBehalfEnabled $true

Option 2: Registry

HKEY_CURRENT_USER\Software\Microsoft\Office\x.0\Outlook\Preferences

DWORD DelegateSentItemsStyle 1

Note! Outlook must be configured to run in cached mode, otherwise mails will get stuck in the Outbox.

References:

https://learn.microsoft.com/en-us/exchange/troubleshoot/user-and-shared-mailboxes/sent-mail-is-not-saved

One or a few users are not able to connect to their mailbox using the newest Outlook version there is? Well the issue might be TLS 1.2 not being active on the user’s side.

Just set this registry key and you should be good to go:

HKEY_CURRENT_USER\Software\Policies\Microsoft\Windows\CurrentVersion\Internet Settings

or

HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Internet Settings

DWORD SecureProtocols 00000A80

HEX A80 or decimal 2688 enables TLS 1.0, 1.1 and 1.2, while decimal 640 only enables TLS 1.0 and 1.1.

References:

https://www.anoopcnair.com/disable-enable-tls-1-0-1-1-internet-explorer-ie/

In Microsoft 365 / Exchange Online, to connect a soft deleted mailbox, you have need to use the cmdlet New-MailboxRestoreRequest to restore the mailbox to another user.

Get-Mailbox -SoftDeletedMailbox | select guid

Get-Mailbox <NewMailbox> | select guid

New-MailboxRestoreRequest -SourceMailbox <GUID> -TargetMailbox <GUID of new mailbox> -AllowLegacyDNMismatch

To check the status of the restore:

Get-MailboxRestoreRequest | Get-MailboxRestoreStatistics

References:

Reddit

If you need make a room mailbox public, for example for a public event, these few PowerShell commands will help you:

First, connect to Exchange Online with your tenant admin:

Connect-ExchangeOnline

Make the calendar public and make the calendar entries visible to everyone:

Set-MailboxCalendarFolder publicRoomMailbox:\Calendar -PublishEnabled $true

Set-MailboxCalendarFolder publicRoomMailbox:\Calendar -Detaillevel Full

Show the HTML and iCalendar URL:

Get-MailboxCalendarFolder publicRoomMailbox:\Calendar | fl published*

The URLs can be shared with anyone or the HTML URL could be embedded on a website as part of an iFrame.

If you need to change a synced AD account with an Exchange Online mailbox into a cloud-only user, these are the steps you need to take:

Move the user to an OU that is not synced by AD Connect and run a delta sync on the AD Connect server (or wait for 30 minutes):

Start-ADSyncSyncCycle -PolicyType Delta

The mailbox is now in the “Soft deleted” state, which means that it is deleted but can be restored within 30 days. To view the soft deleted mailbox run this command:

Connect-ExchangeOnline

Get-Mailbox -SoftDeletedMailbox user@ajni.it | fl guid

Now link the deleted mailbox with the cloud-only user. The cloud-only user has already been created in my case. You will need to insert the user’s password in order to proceed.

New-Mailbox -InactiveMailbox 7fb16ffd-cf3a-42ef-a9d3-e0293f5ef6c2 -Alias cloudOnlyUser -MicrosoftOnlineServicesID cloudOnlyUser@ajni.onmicrosoft.com

Afterwards you can either assign a license to the user or convert that mailbox into a shared mailbox which does not need a license.