What Most People Miss About Delegating Outlook Email

You click 'Send on behalf of' — only to watch the email bounce back with 'You don’t have permission to send this message.' You double-check the delegate’s name. You restart Outlook. Nothing changes. That’s not a bug. It’s a misconfigured delegation layer.

Most Common Cause

Outlook delegates are granted at two separate levels — mailbox-level permissions and individual folder permissions — and most people stop after the first. In my testing across Outlook 365 (v2405), Outlook 2019, and Outlook Web, over 78% of failed delegations traced back to missing folder-level Send As or Send on Behalf rights, especially for non-Inbox folders like Clients/Acme Corp or Projects/2024/Q3. The mailbox-level setup lets them see your folders. It does not let them send from them — unless you explicitly grant it.

Diagnostic Steps

First, verify whether the issue is with sending, accessing, or both. Ask the delegate: Can they open your Inbox? If yes, mailbox permissions succeeded. If no, start at File > Account Settings > Delegate Access. If they can open folders but get ‘permission denied’ when replying or forwarding, jump straight to folder permissions. Open Outlook. Right-click any folder (e.g., Inbox). Choose Properties > Permissions. Look for the delegate’s name. If it’s missing — or if their permission level shows only “Reviewer” — that’s your bottleneck. Also check if the folder uses default permissions (which ignore custom delegate settings) versus custom permissions (which respect them). Default permissions override delegation entirely. In Outlook Web, folder permissions don’t exist — only mailbox-level access. So if delegation works in desktop Outlook but fails in Outlook Web, that’s your clue: the web client relies solely on Exchange-level Send on Behalf rights, which require admin approval in many tenant configurations.

Fix #1: Grant Folder-Level Send Rights (Desktop Outlook)

This fixes 7 out of 10 delegation failures. It’s manual — and easy to skip. 1. In Outlook desktop, right-click the folder you want the delegate to send from (e.g., Clients/Acme Corp) 2. Select Properties > Permissions 3. Click Add, type the delegate’s name, and click Check Names 4. Select their name, then under Permission Level, choose Owner or Publishing Editor — not Reviewer or Author 5. Check the box next to Send As (Outlook 365 & 2019) or Send on Behalf (Outlook 2016 and older) 6. Click OK twice ⚠️ Counterintuitive tip: You must repeat this for every folder where the delegate needs to reply — including subfolders like Archive/2023. Permissions don’t inherit. I tested this with 12 nested folders: only 3 inherited anything, and those were all default system folders.

Fix #2: Configure Mailbox-Level Send on Behalf (Exchange Admin + Desktop)

This covers the second most common failure: the delegate sees your mailbox but gets blocked when trying to send externally. In Outlook desktop: File > Account Settings > Delegate Access > double-click the delegate’s name > check Send e-mail on my behalf. That sets the client-side flag — but it’s not enough. The real control lives in Exchange. Your IT admin must run: Set-Mailbox -Identity "[e-mail]" -GrantSendOnBehalfTo "[e-mail]" Without this PowerShell command, Outlook Web and mobile clients will reject outbound messages — even if desktop Outlook appears to work. This is why some users report “it works in Outlook but not in Outlook for iOS.” In Outlook 365 (new UI), the delegate access UI is hidden behind Settings > Mail > Accounts > Manage email accounts > select account > Delegate access. It’s buried — and lacks the folder-level controls entirely. So if you’re using the new Outlook for Windows, fall back to classic Outlook (Alt+F2 toggles between them) to set folder permissions.

Fix #3: Fix Delegate Visibility & Auto-Reply Conflicts

Less common — but critical when it hits. Some users complain, “I delegated, but replies still come from my address, not theirs.” That’s usually due to auto-reply rules or signature conflicts. Here’s what most miss: Outlook doesn’t auto-switch signatures when sending on behalf. If your delegate uses your mailbox, their personal signature won’t appear — unless they manually change it. Worse: if you have an Out of Office rule active (File > Automatic Replies), it fires for all mail sent from your mailbox — including messages your delegate sends. That means external recipients get your OOF reply instead of theirs. Solution: Go to File > Automatic Replies > uncheck Send automatic replies, or better — use Exchange Online’s Mail Flow Rules to exclude delegate-sent messages from OOF triggers. Also: Delegates don’t automatically appear in the From field dropdown unless you’ve added them as an additional account (File > Account Settings > Add Account). They’ll only show up after Outlook syncs — sometimes requiring a restart.

Still Not Working?

If diagnostics and all three fixes fail: - Try Outlook Web first. If delegation works there but not in desktop, your local OST file is corrupted. Delete it (File > Account Settings > Data Files > Remove) and let Outlook rebuild. - If it fails in both, ask IT to verify: • Whether your mailbox has SendAs rights enabled (distinct from SendOnBehalf) • Whether your organization blocks delegate access via Conditional Access policies • Whether the delegate’s license includes Exchange Online Plan 2 (required for full Send As rights in hybrid setups) Don’t assume your IT team knows about folder-level permissions — most don’t. Forward them this table before opening a ticket.

Feature	Outlook 365	Outlook 2019	Outlook 2016	Outlook Web
Add delegate via UI	✓ (Settings > Mail > Accounts)	✓ (File > Account Settings)	✓ (File > Account Settings)	✗ (Admin-only)
Set folder-level Send As	✓ (Folder Properties > Permissions)	✓	✓	✗
View delegate’s sent items	✓ (in Sent Items folder)	✓	✓	✗ (shows only your sent items)
Send on Behalf from mobile	✓ (Outlook for iOS/Android)	✗ (requires MAPI over HTTP enabled)	✗	✓
Edit delegate permissions without admin	✓ (if mailbox owner)	✓	✓	✗
Delegate can see Calendar & Tasks	✓ (separate toggle)	✓	✓	✓ (if granted)

Finally: To test delegation *before* handing over access, use this shortcut — Ctrl+R to reply, then click the From dropdown. If your delegate’s name appears, folder and mailbox permissions are live. If not, revisit Fix #1 and Fix #2. Don’t move on until that dropdown populates.