Understanding Email Forwarding in Exchange & Microsoft 365: ForwardingSmtpAddress vs. ForwardingAddress

Email forwarding is a fundamental capability within Exchange and Microsoft 365 environments, yet the distinction between ForwardingSmtpAddress and ForwardingAddress often causes confusion. Let's clarify these two parameters and how they govern email redirection.

Understanding the Core Parameters

1. ForwardingSmtpAddress: Directing Emails Externally

The ForwardingSmtpAddress parameter is your go-to for sending emails to recipients outside your organization. Think of personal email accounts like Gmail, Yahoo, or any other external domain. Since these are external addresses, they don't require the same internal validation as recipients within your own network.

  • Use Case: Imagine a contractor, Jane, who needs all emails sent to her company address to automatically land in her personal Gmail inbox (jane.doe@gmail.com). Setting her mailbox's ForwardingSmtpAddress to this external address achieves this seamlessly.

2. ForwardingAddress: Keeping Email Redirection Internal

In contrast, the ForwardingAddress parameter is strictly for forwarding emails to recipients within your organization. These internal recipients are validated and can be identified using various internal identifiers like their alias, distinguished name, or GUID.

  • Use Case: Consider a scenario where Mark is leaving the company, but his manager, Sarah, needs to monitor his incoming mail. You would configure Mark's mailbox with the ForwardingAddress set to Sarah's internal mailbox (sarah@company.com).

Controlling Delivery Behavior: DeliverToMailboxAndForward

The DeliverToMailboxAndForward parameter offers crucial control over whether the original mailbox retains a copy of the forwarded email.

  • Setting DeliverToMailboxAndForward to $true means the email is delivered to the original recipient's mailbox and forwarded to the specified forwarding address.

  • Setting it to $false means the email is only forwarded to the designated address and does not appear in the original recipient's inbox.

  • Use Case Example: If Mark's mailbox is set to forward emails to Sarah, and DeliverToMailboxAndForward is $true, emails will appear in both Mark's inbox and Sarah's. If it's $false, only Sarah will receive the emails.

The Precedence Rule: Internal Over External

A critical behavior to understand is that when both ForwardingSmtpAddress and ForwardingAddress are configured on a mailbox, ForwardingAddress takes precedence. The system prioritizes internal forwarding over external forwarding.

  • Scenario: If Jane's mailbox is configured with both her external Gmail address (ForwardingSmtpAddress) and Sarah's internal address (ForwardingAddress), any email sent to Jane's mailbox will only be forwarded to Sarah. The external forwarding to Gmail will be ignored.

Configuring Email Forwarding

You have several methods for configuring email forwarding in Exchange and Microsoft 365:

  • Outlook on the Web (OWA): Typically used for setting ForwardingSmtpAddress for individual users. You can usually find this under Settings > Mail > Forwarding.
  • Exchange Admin Center (EAC): Provides granular control. Within a mailbox's properties under the Email Forwarding tab, you can configure both ForwardingAddress (for internal) and ForwardingSmtpAddress (for external).
  • Microsoft 365 Admin Center: When setting up email forwarding through the user interface here, it generally configures the ForwardingSmtpAddress.

Why This Distinction Matters

A solid understanding of these parameters is vital for efficient administration and troubleshooting:

  • Transport Rules Interaction: Be aware that when ForwardingSmtpAddress is set via EAC, certain transport rules might not process messages as expected. For instance, rules based on the X-MS-Exchange-ForwardingLoop header might fail because this header is often added after transport rules have evaluated the message.
  • Tenant Migration Scenarios: During a tenant-to-tenant migration, leveraging ForwardingSmtpAddress with the Set-Mailbox PowerShell cmdlet on the old mailbox can ensure a copy of incoming mail continues to be delivered to the new mailbox during the transition period.
  • Remote Domain Settings: The functionality of ForwardingSmtpAddress is subject to the AutoForwardEnabled setting configured on the relevant Remote Domain. If auto-forwarding is disabled for a remote domain, external forwarding to that domain via ForwardingSmtpAddress will not work.

By understanding these key parameters and their behaviors, you can effectively manage email forwarding within your Exchange and Microsoft 365 environment, ensuring mail is routed correctly and predictably.

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