Start with one question: is the message sitting in your Outbox, or did it never leave the compose window? That single distinction splits Outlook sending failures into two mostly separate families of problem, and it saves you from trying random fixes for twenty minutes.

Message stuck in the Outbox means Outlook accepted it and can’t hand it to the server — a connection, authentication, or attachment-size issue. Message that won’t leave the compose window at all, or throws an error the instant you hit Send, usually means an add-in, a corrupt profile, or a bad recipient entry.

Here’s the thirty-second version before the detail: check Send/Receive > Work Offline and make sure it isn’t lit up. That toggle gets hit accidentally more than any other setting in Outlook, and it causes exactly this symptom with no error message whatsoever.

The four checks that resolve most cases

Run these in order before anything else. Together they account for the large majority of “my email won’t send” calls.

1. Is Outlook in Offline Mode?

Look at the status bar bottom-right. If it says “Working Offline” or “Disconnected,” go to the Send / Receive tab and click Work Offline to toggle it back on. The button is highlighted when offline mode is active.

Outlook also drops into offline mode by itself after a network interruption and doesn’t always come back cleanly. If toggling doesn’t restore the connection, close Outlook completely and reopen it.

2. Is there a huge attachment blocking the queue?

Open the Outbox folder and check the size of each stuck message. Microsoft 365 caps messages at 25 MB by default, and MIME encoding inflates attachments by roughly a third — so a 20 MB file becomes about 27 MB on the wire and gets rejected.

This is the sneaky one, because the oversized message doesn’t just fail. It sits at the front of the queue retrying, and every legitimate email behind it waits. You’ll swear nothing is sending when in fact one message from an hour ago is holding the door.

Delete it and everything behind it flushes. To delete a stuck message you may need to go offline first (Send/Receive > Work Offline), delete it, then go back online — Outlook locks messages it’s actively trying to transmit.

For anything over 15 MB, share a OneDrive or SharePoint link instead. Faster, and it doesn’t count against anyone’s mailbox quota.

3. Does the same account work in the browser?

Log into outlook.office.com (or your provider’s webmail) and send a test message. This is the single most useful diagnostic available and it takes a minute.

Works in the browser but not the desktop app? The problem is local — profile, add-in, OST, or settings. Fails in the browser too? The problem is the account or the server, and no amount of desktop troubleshooting will help. Check whether the mailbox is full, whether the account is suspended, or whether your domain landed on a blocklist.

4. Does it work in safe mode?

Close Outlook. Press Win+R, type outlook.exe /safe, press Enter. This launches Outlook with all add-ins disabled.

If sending works in safe mode, an add-in is the culprit. Restart normally and go to File > Options > Add-ins > Manage: COM Add-ins > Go. Untick everything, restart, confirm sending works, then re-enable them one at a time until it breaks again.

Usual suspects in small business environments: CRM connectors, e-signature toolbars, antivirus email plugins, and older Adobe integrations. Antivirus add-ins are the most frequent offender by a distance.

Error code reference

Symptom: message sits in the Outbox and never moves

Beyond the attachment issue above, three things cause this.

Send/Receive is set to manual. Go to Send / Receive > Send/Receive Groups > Define Send/Receive Groups. Confirm “Schedule an automatic send/receive every [n] minutes” is ticked and that “All Accounts” is included in the group. If someone unticked this — sometimes done deliberately on a laptop to save battery — Outlook only sends when you press F9.

The message is open somewhere. Outlook won’t transmit an item that’s open in another window, and it also won’t transmit if you’ve clicked into the Outbox and highlighted the message. Click into a different folder entirely, then press F9.

A bad recipient address. Outlook’s autocomplete cache can hold a malformed address from months ago that fails every time it’s used. Delete the stuck message, compose a new one, and type the recipient address manually rather than picking the autocomplete suggestion. To clear the cache entirely: File > Options > Mail > Empty Auto-Complete List.

Symptom: it says “Sending” forever

The progress bar crawls or hangs. Outlook is talking to the server but the exchange isn’t completing.

First suspect: antivirus email scanning. Norton, McAfee, Avast, AVG, and ESET all offer a module that intercepts SMTP traffic and inspects it before release. On a large message, or when the scanning engine stalls, the send appears to hang indefinitely.

Turn off the email scanning component specifically — not the entire antivirus product — and retry. If that fixes it, leave it off. Mail is encrypted in transit with TLS these days, so the scanner has to break and re-establish the connection to see anything, and it inspects attachments again when they’re written to disk regardless. You’re giving up very little.

Second suspect: an oversized OST. If your data file has grown past 20–30 GB, Outlook slows down across the board. Check File > Account Settings > Account Settings > Data Files for the file location, then check its size in Explorer. Reduce the cached mail window: File > Account Settings > Account Settings > double-click the account > drag “Mail to keep offline” down to 3 or 6 months. Everything older stays on the server and remains searchable online.

Symptom: it works on my phone but not on my desktop

This one is genuinely informative. Phone works means the account, password, and server are all fine. The problem is entirely in the desktop Outlook installation.

It also very often means authentication method. Phone mail apps use OAuth by default. Older desktop Outlook profiles may still be configured for basic authentication, which Microsoft has been progressively disabling across Microsoft 365 tenants. When basic auth gets switched off for your tenant, the phone keeps working and the desktop stops.

The fix: rebuild the mail profile

A fresh profile picks up modern authentication automatically. Close Outlook, then:

• Control Panel > Mail (Microsoft Outlook). If you can’t find it, switch the view to Large icons — it hides in Category view.
• Click Show Profiles, then Add. Name it something like “New2026”.
• Enter your name, email address, and password. Let autodiscover configure it.
• Select Always use this profile and choose the new one.
• Open Outlook and let it sync.

Leave the old profile in place until you’re confident the new one is complete. Signatures, rules, and autocomplete entries don’t carry across automatically — signatures live in %APPDATA%MicrosoftSignatures and can be copied. Rules can be exported from the old profile via Rules > Manage Rules & Alerts > Options > Export Rules.

For an Exchange or Microsoft 365 mailbox this is safe — everything lives on the server. For a POP3 account, do not do this without first confirming where your mail is stored locally. POP mail may exist only in that PST file.

Authentication and app passwords

If Outlook keeps prompting for your password and rejecting the correct one, you’re almost certainly hitting multi-factor authentication.

When MFA is enabled on an account, applications that can’t display the second-factor prompt need an app password — a long generated string used in place of your real password by that one application. Your normal password will be rejected no matter how carefully you type it.

For a Microsoft work or school account, app passwords have to be enabled by an administrator first (Microsoft Entra admin center > Security > Authentication methods). Once available, users generate one at mysignins.microsoft.com/security-info > Add sign-in method > App password.

Better path where possible: get the account onto modern authentication so app passwords aren’t needed at all. On current Outlook builds connected to Microsoft 365, the client handles MFA natively with a browser popup. If you’re seeing repeated password prompts on a current build, the profile is probably stale — rebuild it as above.

Server settings worth verifying

For manually configured IMAP/POP accounts — anything not Microsoft 365 or Gmail auto-setup — wrong outgoing settings are a common cause. File > Account Settings > Account Settings > double-click the account > More Settings > Outgoing Server and Advanced tabs.

What you should generally see:

• SMTP port 587 with STARTTLS — the modern standard for submitting mail
• Port 465 with SSL/TLS — still widely supported, fine if your host specifies it
• Port 25 — blocked by most ISPs for residential and many business connections. If you’re on 25, that’s likely your problem
• “My outgoing server (SMTP) requires authentication” must be ticked, set to use the same credentials as the incoming server

To test whether the port is reachable at all, from Command Prompt:

Test-NetConnection smtp.office365.com -Port 587

Run that in PowerShell. TcpTestSucceeded : True means the network path is clear and the problem is elsewhere. False means a firewall, router, or ISP is blocking outbound SMTP.

Repairing a corrupt data file

Error 0x8004010F and general flakiness both point at the OST or PST. Close Outlook and run SCANPST.EXE from:

C:Program FilesMicrosoft OfficerootOffice16SCANPST.EXE

Browse to your data file, click Start, let it scan, and tick “Make a backup of scanned file” before repairing. It may need multiple passes — keep running it until it reports a clean file.

For Exchange and Microsoft 365 accounts there’s a faster route. The OST is a local cache; the authoritative copy is on the server. Close Outlook, rename the .ost file to .ost.old, reopen Outlook, and it rebuilds. A large mailbox may take an hour or more to resync, so start it before lunch, not before a client meeting.

Common questions

Why does Outlook say a message sent but the recipient never got it?

That’s a delivery problem, not a sending problem — different diagnosis entirely. The message left your machine and something downstream discarded it. Check the recipient’s spam folder first, then verify your domain’s SPF, DKIM, and DMARC records at mxtoolbox.com. Missing or misconfigured records are the most common reason mail from small business domains gets silently dropped by Google and Microsoft.

Can I just delete everything in the Outbox?

Yes, and often you should. Anything in the Outbox has not been delivered, so deleting it loses nothing that was received. Copy the body text of anything you want to keep first. If a message won’t delete, switch on Work Offline, delete it, then go back online.

Does the new Outlook for Windows have these problems?

Some of them go away. The new Outlook is browser-based, so there’s no OST to corrupt, no COM add-ins, and no mail profile to rebuild. But it loses features that classic Outlook has — certain add-ins, PST support, some offline capability — so it isn’t automatically an upgrade for a business that relies on those. Test on one machine before moving everybody.

How large can an attachment be?

Microsoft 365 defaults to 25 MB, adjustable up to 150 MB by an admin. Gmail is 25 MB. But the limit that matters is the recipient’s, and you can’t see it. Anything over about 10 MB is safer sent as a link. Encoding overhead also means a 24 MB file can exceed a 25 MB limit once it’s on the wire.

Sending stopped working right after a Windows update. Coincidence?

Usually not. Updates can reset network stack settings, re-enable a firewall rule, or break an add-in’s compatibility. Run sfc /scannow from an elevated prompt, then test Outlook in safe mode with outlook.exe /safe. If safe mode works, an add-in stopped being compatible with the new build and needs updating or removing.