Outbound Recipient Limits

Question

Hi

In a Hybrid exchange environment from what I can see there seems to be multiple ways to set outbound mail recipient limits.

On Outbound spam policy allows us to set a limit and we can target a mail enabled group to apply to specific users. I believe the default sets this limit to 10,000 emails.

We can also via the EAC apply a recipient limit on one or more users. and we can Specify the maximum number of allowed recipients on the To, Cc and Bcc lines of an email message sent from this mailbox, from 1 to 1000

Are these pretty much doing the same thing? There is potential for conflict there.

Also what is the best way to set tenant level recipient limit? The article below describes how to do this would this also be applicable to a Hybrid environment?

https://techcommunity.microsoft.com/t5/exchange-team-blog/customizable-tenant-level-recipient-limits/ba-p/1571564

I assume if it is valid then setting that would overwrite the other methods as its a tenant level setting?

Answer 1

The outbound spam policy only applies to messages sent externally, not to internal mailboxes.
its also important to note that the outbound SPAM policy applies ONLY to 365 mailboxes, not on-prem mailboxes you may have in a hybrid config
https://learn.microsoft.com/en-us/microsoft-365/security/office-365-security/configure-the-outbound-spam-policy?view=o365-worldwide

The mailbox recipient limit in EAC applies to ALL messages, internal or external for just that mailbox where its set:

The RecipientLimits parameter specifies the maximum number of recipients allowed in messages sent by the mailbox.
In on-premises Exchange, a valid value is an integer or the value unlimited. The default value is unlimited, which indicates the maximum number of recipients per message for the mailbox is controlled elsewhere (for example, organization, server, or connector limits).
In the cloud-based service, a valid value is an integer from 1 to 1000.

https://learn.microsoft.com/en-us/powershell/module/exchange/set-mailbox?view=exchange-ps

The tenant level setting you linked to only applies if the individual mailbox setting is set to unlimited, otherwise the mailbox setting applies.

If the mailbox or mailuser RecipientLimits property value
Is a numeric value then
Use that value as the maximum number of recipients a sender can send to per message
Else if the mailbox or mailuser RecipientLimits value is “Unlimited” then
Use the tenant-level setting (the value on the MaxRecipientEnvelopeLimit property on the tenant’s TransportConfig) instead
If the tenant-level setting is also “Unlimited” then
Use the Exchange Online service-level setting (1000 as of this writing)

Note that since it applies to both usermailboxes and MailUsers, then the tenant level should apply in hybrid as well since on-prem UserMailboxes are sycned as MailUsers in Exchange Online.
If you want to set this at the tenant level and apply to everyone for both internal and external messages, then clear any per mailbox limits and set the tenant level as you require

Answer 2

Brilliantly explained and fully understood. Thank you sir.

Answer 3

From my testing there is another difference between the outbound spam policy and using the EAC to set a limit.

With the EAC is only affects maximum amount of recipients per email.

If I set a limit of 3 I can send 4 separate emails for example. I cannot however have 4 recipients in the one email.

With an outbound spam policy the limit is both on recipients per email and per email. So If I set a limit of 3 I cannot send 4 separate emails and I cannot send the one email to 4 recipients.

Just something worth noting