Customizable Tenant-level Recipient Limits

Controlling the number of recipients per message that your users can send to is one of several measures email admins can use to help curtail the risk of email abuse and spamming from compromised accounts. Earlier this year the Exchange Online Transport team introduced customizable Recipient Limits in Exchange Online. With it you can customize the setting on a mailbox or mail user object, you can bulk edit multiple existing mailboxes or mail users at the same time, and you can even set the default for new ones. Yet, that update didn’t offer a single, master tenant-wide setting. It gave Exchange Online admins more control than they had before, but it still wasn’t as comprehensive as what Exchange Server on-premises offers.

Today we’re announcing the immediate availability of a super useful extension of the mailbox customizable recipient limits feature that helps deliver a more complete solution: The ability to use Remote PowerShell to customize a tenant-wide recipient limit with the MaxRecipientEnvelopeLimit property on the TransportConfig object (Set-TransportConfig -MaxRecipientEnvelopeLimit).

Similar to how it works in Exchange Server on-premises, the MaxRecipientEnvelopeLimit property is the authoritative or fall back setting for recipient limits when a mailbox’s or mail user’s RecipientLimits property is set to “Unlimited”. Now, just because it says ”Unlimited” doesn’t mean that it is. It actually means “fall back to the next higher level setting,” which for a mailbox or mail user is to fall back to the value on the tenant-level setting, the tenant’s TransportConfig MaxRecipientEnvelopeLimit setting. And “Unlimited” on the tenant-level setting, in turn, means to “fall back to the Exchange Online service-level setting” – which in the Exchange Online multi-tenant environment is currently 1,000 recipients.

Here are the guiding principles in pseudo-code style that the Exchange Online service uses to determine the maximum number of recipients a sender can send to per message:

• 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: It’s easy to fall into the mistake of thinking that the tenant-level setting is the most-restrictive setting regardless of what the mailbox or mailuser setting is. But that’s not true. As shown above, the only time the tenant-level setting is used is if the mailbox or mailuser setting is “Unlimited.” This is the same as how it works in Exchange Server on-premises.

Here are several examples of how this works:

So, managing recipient limits in Exchange Online now works pretty much like it does in Exchange Server on-premises. Do note that one significant difference is that while the default MaxReceipientEnvelopeLimit for new tenants in Exchange Online will still be “Unlimited”, the allowable range for customizing it in Exchange Online is 1 to 1000, while in Exchange Server on-prem the allowable range is from 0 to 2147483647.

While it might appear to be a minor update, we believe this change will prove to be quite valuable for email admins so they can more simply and flexibly manage and control a variety of recipient limits scenarios. It’s a new Exchange Online capability that continues to demonstrate our commitment to delivering more control, more “knobs and dials,” to manage your organization’s mail flow. Let us know what you think!

Kevin Shaughnessy