Configurable “On Behalf Of” From Address for Multi-Tenant Email Sending

Hi James, thanks for the thoughtful breakdown — genuinely appreciate the clarity around From, Sender, DMARC alignment and how Outlook chooses to render “on behalf of” relationships.

That said, this still doesn’t address the core problem we’re raising.

Our requirement isn’t unusual or niche. Many mainstream email providers that support SaaS platforms at scale — sending thousands or millions of messages on behalf of clients — already offer a configurable dual‑identity model where:

the client’s domain is the From identity

the platform’s domain is the authenticated Sender identity

email clients display the native “on behalf of / via” relationship

This is standard behaviour in other ecosystems, and it’s essential for multi‑tenant SaaS platforms like ours where transparency, branding clarity, and trust are critical.

Your answer confirms that ACS currently cannot provide this dual‑identity configuration and instead suggests workarounds that either break the branding model or require heavy domain‑by‑domain onboarding that doesn’t scale for large SaaS environments.

So while I appreciate the explanation, the reality is that ACS is missing a capability that other providers already support and that SaaS platforms rely on. This is why we’re asking Microsoft to consider native support for this pattern — not as a workaround, but as a first‑class feature.

Thanks again for engaging with the question.

Hola Pamela,

I think what you’re trying to achieve makes sense, especially for a multi-tenant platform where the recipient needs to understand both who the message is for and which platform actually sent it.

The tricky part is that there are a couple of different email concepts involved here.

The From header identifies the apparent author of the message. A separate Sender header can identify the system or account that actually transmitted it when it differs from the From identity. Some mail clients, including Outlook, may display that relationship as “on behalf of” or “via,” but that presentation is controlled by the receiving client. It isn’t simply text that Azure Communication Services can force Outlook to display.

ACS Email does support custom verified domains, multiple sender usernames, and configurable display names.

So, for example, if mg.businessname.com is verified in ACS, you could send from: ******@mg.businessname.com

Display name: Business Name (via BizPlex)

That would give you the customer-specific branding you’re looking for.

Where ACS appears to fall short is the exact dual-identity model you described. It doesn’t currently give you a per-message way to say “this business is the From identity, but BizPlex is the separate Sender identity” and then rely on Outlook to render that as a native “on behalf of” relationship.

DMARC is also important here. DMARC evaluates alignment against the domain in the From address, so if the message says it is from mg.businessname.com, that domain needs to be properly authenticated with an aligned SPF and/or DKIM record. Simply authenticating bizplex.com while putting the customer’s domain in the From field would not give you the clean alignment you’re after.

Microsoft explains the ACS authentication model here.

Practically, I think you have two workable options today:

Use a verified sending domain or subdomain for each customer. That gives you the strongest customer branding and proper authentication, but it adds some onboarding work.

Or, send from a BizPlex-owned domain and use a display name such as “Business Name (via BizPlex),” with Reply-To set to the customer’s address. That’s much easier to operate at scale, but the actual From domain remains BizPlex.

If your requirement is specifically for Outlook to show a true “Business Name on behalf of BizPlex” relationship with both identities represented separately, I believe that would need to be treated as a feature request for ACS rather than something that can be configured today.

Thanks,

James