Ah, yes - that's a subtle but common gotcha in Microsoft 365. Let's break it down carefully.

When you convert a user mailbox to a shared mailbox:

1. The mailbox itself remains but shared mailboxes do not have their own login for sending/receiving mail in the same way a regular user mailbox does.

2. If you leave the original user account active but remove the mailbox license (or convert it to shared), the mailbox can no longer receive email directly from the outside unless:

   • It still has a valid Exchange Online license (needed if it will receive mail).
   • Or the receiving permissions are properly set up for delegates.

Here's why your mailbox receives no mail now:

• When you convert a regular mailbox to shared, Microsoft automatically removes the license associated with the mailbox because shared mailboxes don't require a license if under 50GB.
• Without a license, the mailbox cannot receive mail from external senders. Internal sending (within your organization) may still work if permissions are correct.

How to fix it

Option 1: Keep it as shared mailbox but allow external mail

1. Go to the Microsoft 365 admin center Users Active users select the original account.
2. Check if the mailbox has a license. If you removed it, reassign an Exchange Online license temporarily.
3. Ensure email forwarding / mailbox permissions are set up correctly for any delegate users.

Option 2: Restore as a normal user mailbox

• If this mailbox needs to receive mail directly and possibly login, you might need to reconvert it to a regular user mailbox. This requires:

  1. Assigning a full Exchange Online license.
  2. Ensuring the mailbox is not marked shared anymore.

Option 3: Keep shared but redirect mail

• If the mailbox must remain shared, you can set up a mail flow rule (forwarding) or alias from the old address to a licensed mailbox so mail is delivered.

Important subtlety: Simply converting a mailbox to shared does not automatically break mail delivery, but if the license is removed or the mailbox doesn't have a valid recipient type for external mail, messages will bounce or disappear.

If you want, I can give you step-by-step commands in PowerShell to check why this mailbox isn't receiving mail and fix it without touching users too much.

Do you want me to do that?

Ah - now that's an important clarification. Yes, shared mailboxes normally don't need a license to receive mail from external senders, up to 50GB, and without an associated user account. So why is yours broken? Let's dissect the difference.

Here's what's likely happening in your case:

1. Original admin mailbox scenario:

   • You had a user mailbox that was also the global admin login.
   • You converted it to shared, but the original user account still exists.
   • Microsoft 365 sees this as a shared mailbox with a conflicting user account, and sometimes this blocks mail delivery, especially if the user account still has a primary SMTP address that matches the mailbox.

2. Other shared mailboxes work because:

   • They are pure shared mailboxes, not "converted from a user account" that still exists.
   • There is no conflicting user login with the same primary SMTP address.

3. Potential catch:

   • When you converted the admin mailbox to shared but left the user account intact, the user account may still be "owning" the SMTP address internally, preventing mail routing to the shared mailbox.
   • If the user account was disabled or unlicensed, the mailbox may appear "orphaned" in terms of delivery routing.

How to fix it

Option 1: Remove the primary SMTP from the original user account

1. Go to Microsoft 365 admin center Users select the original user account.
2. Edit email addresses and remove the primary SMTP (or change it temporarily to something else).
3. Ensure the shared mailbox has the correct primary SMTP.

Option 2: Make sure the shared mailbox is set to receive external mail

1. In Exchange admin center Mailboxes select shared mailbox Mail Flow Settings Delivery Management.
2. Ensure it is enabled to receive mail from all senders (internal + external).

Option 3: Check mail routing in PowerShell

You can quickly check the mailbox type and email addresses with:

Connect-ExchangeOnline
Get-Mailbox -Identity "adminshared"

• Look for RecipientTypeDetails (should be SharedMailbox)
• Look for PrimarySmtpAddress
• Look for EmailAddresses
• Check if the mailbox is hidden from address lists (hidden mailboxes sometimes reject external mail).

The core issue: Shared mailboxes converted from a user who still exists sometimes have SMTP ownership conflicts, unlike "pure" shared mailboxes. That's why your other shared mailboxes work fine.

If you want, I can give you the exact sequence of PowerShell commands to fix this mailbox so it receives mail immediately, without touching your admin user login.