Get-EXOMailbox -ResultSize Unlimited: return more than 1000 mailboxes

Get-EXOMailbox -ResultSize Unlimited returns every mailbox that matches the query. Without the parameter, the cmdlet stops after 1000 results: the reference says "The ResultSize parameter specifies the maximum number of results to return. If you want to return all requests that match the query, use unlimited for the value of this parameter. The default value is 1000." In any tenant with more than 1000 mailboxes, a report without it is silently incomplete.

Connect-ExchangeOnline -UserPrincipalName admin@contoso.com
Get-EXOMailbox -ResultSize Unlimited | Measure-Object
Get-EXOMailbox -RecipientTypeDetails SharedMailbox -ResultSize Unlimited |
  Select-Object DisplayName, PrimarySmtpAddress |
  Export-Csv shared.csv -NoTypeInformation

Accepted values

The parameter type is Unlimited[UInt32]: either the word Unlimited or a whole number from 0 to 4294967295. Checked offline in Windows PowerShell 5.1 with ExchangeOnlineManagement 3.10.1, before any connection:

Import-Module ExchangeOnlineManagement
$t = (Get-Command Get-EXOMailbox).Parameters['ResultSize'].ParameterType
"Type: " + $t.Name + " of " + $t.GenericTypeArguments[0].Name
$u = [System.Management.Automation.LanguagePrimitives]::ConvertTo('Unlimited', $t)
"'Unlimited' -> IsUnlimited=" + $u.IsUnlimited
$n = [System.Management.Automation.LanguagePrimitives]::ConvertTo('5000', $t)
"'5000' -> IsUnlimited=" + $n.IsUnlimited + ", Value=" + $n.Value
try { Get-EXOMailbox -ResultSize all -ErrorAction Stop } catch { $_.Exception.Message }
try { Get-EXOMailbox -ResultSize Unlimited -ErrorAction Stop } catch { $_.Exception.Message }

Type: Unlimited`1 of UInt32
'Unlimited' -> IsUnlimited=True
'5000' -> IsUnlimited=False, Value=5000
Cannot bind parameter 'ResultSize'. Cannot convert value "all" to type "Microsoft.Exchange.Management.RestApiClient.Unlimited`1[System.UInt32]". Error: "Input string was not in a correct format."
You must call Connect-ExchangeOnline before calling any other cmdlet.

The value is case-insensitive (unlimited works). Words such as All or -1 are rejected when the parameter is bound, before the cmdlet contacts the service; a valid value without a session fails with You must call Connect-ExchangeOnline before calling any other cmdlet.

Pitfalls

• Combine it with a filter. -ResultSize Unlimited with no filter downloads every mailbox. Use -Filter "Department -eq 'Sales'" or -RecipientTypeDetails UserMailbox so the service does the narrowing.
• Ask only for the properties you need. Get-EXOMailbox returns a minimum property set by default; add the few you need with -Properties or a property set with -PropertySets (-PropertySets) instead of -PropertySets All over the whole tenant.
• Same rule in other cmdlets. Get-Mailbox, Get-Recipient and Get-EXORecipient also default to 1000; see Get-EXOMailbox vs Get-Mailbox.
• Permissions. The account needs an Exchange role that includes the cmdlet, such as View-Only Recipients, or the Exchange Administrator role in Microsoft Entra ID.

FAQ