Mailbox Existence

If a recently deleted user account's mailbox is not visible in the Exchange Admin Center (EAC), it does not necessarily mean that the user never had access to Office 365.

Once license is removed mailbox will not appear in EAC, when license is removed, mailbox disconnects from the AD or Azure AD object - by history logs if present we can see if license was every assigned, than we can say it had access before to O365 or not

Below scenarios can be there

1. Mailbox Never Existed

• If the user account never had an Exchange Online license assigned, the user would not have an associated mailbox in Office 365.

2. Mailbox Was Deleted

• If a user had a mailbox but it was deleted (either through account deletion or license removal), it might not appear in the EAC. Mailboxes are retained for a certain period after deletion (30 days by default for Office 365). During this period, the mailbox would appear under "Deleted Mailboxes" in EAC.

3. Hidden from EAC View (rare situation)

• The mailbox could be hidden from the address list (using the HiddenFromAddressListsEnabled attribute) or not visible in EAC due to permissions or sync issues.