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Send As vs Send on Behalf vs sending from an alias in Exchange and Outlook

Send As makes a message appear to come from another mailbox or group; Send on Behalf shows “A on behalf of B”; sending from an alias uses one of the same mailbox’s secondary addresses — a capability Exchange Online offers as a tenant setting and Exchange on-premises does not. The three are often mixed up in support requests because all three change what appears in the From line. They differ in who grants them, what the recipient sees, and where the reply ends up.

Updated on 2026-09-22

The three terms, defined

Send As and Send on Behalf are permissions. An administrator grants them on a mailbox or a group to another user, whom Microsoft calls the delegate. The definitions below follow Microsoft Learn, Manage permissions for recipients (Exchange Server 2016, 2019 and Subscription Edition); the Exchange Online page describes the same two permissions.

• Send As allows the delegate to send messages as if they came directly from the mailbox or group. Microsoft: “There’s no indication that the message was sent by the delegate.” It does not allow the delegate to read the mailbox.
• Send on Behalf allows the delegate to send messages from the mailbox or group, and the From address clearly shows that the delegate sent them: “<Delegate> on behalf of <MailboxOrGroup>”. Replies go to the mailbox or group, not to the delegate. It does not allow the delegate to read the mailbox either.
• Sending from an alias is not a permission at all. A mailbox has one primary SMTP address and can have further addresses, the aliases or proxy addresses. Sending from an alias means the From line carries one of those secondary addresses of the user’s own mailbox.

A fourth term belongs in the same sentence because it is confused with the first two: Full Access lets the delegate open the mailbox and view, add and remove its contents. Microsoft adds that it does not allow the delegate to send messages from the mailbox. A shared mailbox therefore needs Full Access and Send As before anyone can write from it.

Side by side

	Send As	Send on Behalf	Sending from an alias
What the recipient sees	The other mailbox or group. No indication of the delegate.	Both: “<Delegate> on behalf of <MailboxOrGroup>”.	The alias address of the user’s own mailbox.
Where it is granted	By the administrator on the other mailbox or group (EAC mailbox or group delegation, or Add-ADPermission).	By the administrator on the other mailbox or group (GrantSendOnBehalfTo).	Nowhere per user. In Exchange Online it is the organization setting SendFromAliasEnabled; the user then selects the alias in Outlook settings.
Where replies land	In the other mailbox or group.	In the other mailbox or group, not with the delegate.	In the user’s own mailbox — the alias is one of its addresses.
Exchange Server 2016, 2019, SE	Yes.	Yes.	No. Outgoing mail carries the primary SMTP address; there is no equivalent setting.
Exchange Online	Yes.	Yes.	Yes, after the administrator enables SendFromAliasEnabled. Microsoft documents the From dropdown for Outlook on the web; compatible Outlook clients can send from and reply to aliases.
Reads the other mailbox?	No.	No.	Not applicable — same mailbox.

Two rules from the same Microsoft page settle most edge cases. If a user has both Send As and Send on Behalf on the same mailbox or group, Send As is always used. And if the mailbox is hidden from address lists, the delegate cannot send from it with either permission.

What an alias is, and why it is not a permission

Microsoft’s administrator page Add another email alias for a user puts it plainly: the primary e-mail address is what typically appears in the From field when the user sends; the additional addresses of the same account are called aliases, and they receive mail. Nothing has to be granted, because the addresses belong to the user already. The question is only whether the server lets one of them appear as the sender.

In Exchange Online that is decided by one setting. The reference for Set-OrganizationConfig describes SendFromAliasEnabled as the parameter that allows mailbox users to send messages using aliases (proxy addresses), and marks it as available only in the cloud-based service. Its default is $false: aliases on messages sent or received are rewritten to the primary e-mail address. With $true the rewriting stops, and compatible Outlook clients allow sending from aliases and replying to aliases. On the Microsoft 365 page the user-facing side is described for Outlook on the web: after the setting is enabled, each alias appears as a checkbox in the user’s Outlook settings, and a selected alias shows up in the From dropdown of the compose form.

Exchange Server 2016, 2019 and Subscription Edition have no equivalent setting. On premises an alias receives mail and nothing more. That is the whole reason the question “how can my users send from their second address?” has a different answer on-premises than in the cloud.

The reply trap

Even where aliases can be used as senders, the reply is where the From line goes wrong. Microsoft’s alias page describes it for Microsoft 365: when a user answers a message that arrived at an alias, the From address depends on the client — Outlook on the web uses the alias at which the e-mail was received, Outlook desktop uses the primary address. The recipient of that reply then sees a different address than the one they wrote to, and a filter or a contact entry on their side stops matching.

On Exchange Server the same thing happens with every alias, because there is no alias sending at all: whatever address the customer wrote to, the reply leaves with the primary address. An Outlook add-in that sets the sender per message fixes exactly this step. ChangeSender selects the sender address of a reply automatically, using the address the original message was received on, so the conversation keeps the address the customer knows.

On Exchange Server: the choice is Send As or an add-in

When one person has several addresses on premises, two routes remain. The first is built in: each extra address becomes its own recipient object, a mailbox or a distribution group, and the user gets Send As on it. The recipient sees that address, but replies land in the extra object, not in the user’s inbox. The second is an add-in that sets the sender address of the individual message while the address stays an ordinary alias of the mailbox, so replies arrive where they always did. How the built-in ways compare with a shared mailbox and with an add-in, including the commands to grant and verify the permissions, is in the guide How Outlook users can choose a different From address on Exchange on-premises.

When you need an add-in, and what it does

ChangeSender is an Outlook add-in with a server component for Exchange. It adds a send-as selection box to the Outlook mail form, so the From address is chosen per message from the addresses the user is allowed to use. Compared with the two permissions above:

• The message does not show up as “sent on behalf of…”; the chosen address appears as “Any name” <any@address.com>, configurable per address.
• Several users can send from the same address, for example sales@company.com, without a mailbox or group being created for it. For the replies, a distribution group with that address collects them for everyone involved — the steps are in How to use the same sender address for two Exchange mailboxes.
• Signatures change with the selected sender address, sent items can be filed into a folder per address, and keyboard shortcuts (CTRL-1, CTRL-2 and so on) pick the frequent addresses.
• The administrator decides what each user may choose: the addresses from Active Directory (default), one fixed address, or any address. The utility is described in How to restrict users’ choices for Send-As addresses.

Both parts must be installed — the server component on the Exchange server and the add-in on every PC; with the add-in alone the sender address does not change. ChangeSender works with Exchange 2000 through 2019 and Exchange Server SE and with Outlook 2007 through the current Microsoft 365 Apps. The installation guide covers both parts and the download page has both installers.

Which term is the one you need

• The recipient should see that a colleague wrote for someone else: Send on Behalf.
• The message should look as if it came from a team mailbox or a group, with replies going there: Send As.
• One person, several own addresses, replies in their own inbox, correct From on every answer: sending from an alias — a tenant setting in Exchange Online, an add-in on Exchange Server.

Frequently asked questions

More in the Exchange mail flow guides, on the ChangeSender product page or in the knowledge base. Auf Deutsch: Senden als, Senden im Auftrag von oder von einem Alias senden.