The short version
- A message sitting in the Outbox means Outlook accepted it but never handed it to the mail server. The fault is on your PC, your credentials, or your connection — not the recipient's end.
- Check Work Offline first (Send / Receive tab), then attachment size, then whether you've hit a sending rate limit.
- Microsoft 365 caps sending at 30 messages per minute and 10,000 recipients per day. Hit either and Outlook stops sending with no useful error.
- Attachments are re-encoded for transmission and grow by about a third, so an 18 MB file can breach a 20 MB cap.
- Fastest diagnostic: send from webmail in a browser. Works there but not in the app? The problem is your PC, not your account.
If Outlook has stopped sending, the message is sitting in your Outbox for one of six reasons: Work Offline is switched on, the attachment is too big, you've tripped a sending rate limit, the mailbox is full, your saved password or multi-factor token has expired, or the outgoing server settings are wrong. Work through them in that order — the first three take under a minute each to check.
Why is Outlook not sending your emails?
A message in the Outbox means Outlook has accepted it from you but hasn't successfully handed it to the mail server. That's a useful distinction, because it narrows the problem to your computer, your account credentials, or your connection — not the recipient.
Before you change any settings, run this one-minute test: open a browser and sign in to your mail on the web (outlook.com for personal accounts, outlook.office.com for a work or Microsoft 365 account) and send yourself a message from there.
- It sends fine on the web — the problem is the Outlook app on your PC. Settings, profile, cached password, or an add-in.
- It fails on the web too — the problem is the account or the mail server. A full mailbox, a rate limit, a suspended account, or a provider-side issue.
The six usual culprits:
- Work Offline is on — everything queues in the Outbox and nothing moves. Easy to switch on by accident, easy to miss.
- Attachment too large — see the section below, because the number that matters isn't the one File Explorer shows you.
- Sending rate limit hit — the least-known cause, and the one that best explains "it worked this morning". Covered below.
- Mailbox storage full — a full mailbox blocks sending as well as receiving, and the error is rarely obvious.
- Expired login — a changed password or a lapsed multi-factor token. Outlook often keeps showing old mail while silently refusing to send.
- Wrong outgoing (SMTP) server settings — usually only on accounts set up manually years ago.
How do you fix Outlook not sending emails, step by step?
Work through these in order and stop when it sends.
- Turn off Work Offline. Open the Send / Receive tab. If the Work Offline button is highlighted, click it once to go back online, then click Send/Receive All Folders.
- Force a send. Press F9 and watch the status bar at the bottom of the window — an error there tells you far more than a message that just sits still.
- Clear the stuck message properly. Switch Work Offline on first, then open the Outbox and delete or drag the stuck message to Drafts, then switch Work Offline off. Doing it in that order matters — while Outlook is online it holds a lock on a message it's mid-way through sending, and you'll get an error or a second copy.
- Re-enter your password. Go to File > Account Settings > Account Settings, select the account, and click Repair. Being signed in to Windows and being authenticated to your mail server are two separate things — one can be fine while the other has quietly expired.
- Send the big attachment as a link. Delete the file from the message, upload it to OneDrive or Google Drive, and paste a share link instead.
- Check your mailbox isn't full. On the web version, look under settings for storage. If you're near the cap, empty Deleted Items and Junk first — they count.
- Turn off email scanning in your antivirus, temporarily. Just the mail-scanning component, not the whole product. Some third-party security tools sit in the middle of the outgoing connection and time it out. Test, then turn it back on.
- Update Outlook. File > Office Account > Update Options > Update Now.
- Build a fresh profile. Close Outlook, then Control Panel > Mail > Show Profiles > Add. Set the new one as default and re-add your account. A modern Microsoft 365 or Outlook.com account will configure its own server settings correctly when added fresh, which quietly fixes any wrong settings inherited from an old profile.
If you're still stuck after a new profile, the problem is almost certainly server-side or account-side rather than something on your PC — which is the point at which guessing gets expensive and it's worth getting someone to look at the account properly.
Why does a 20 MB attachment fail when the file is only 18 MB?
Because email can't carry raw files. Attachments are re-encoded into text for transmission (MIME/base64), and that encoding inflates them by roughly a third. An 18 MB file arrives at the server as about 24 MB — over the limit, from a file that looked comfortably under it.
The caps that actually apply, as of 2026:
| Where you're sending from | Maximum message size |
|---|---|
| Outlook desktop, POP/IMAP account | 20 MB (a limit inside Outlook itself) |
| Outlook.com personal account | 25 MB |
| Microsoft 365, Outlook desktop | 150 MB |
| Microsoft 365, Outlook on the web | 112 MB |
| Microsoft 365, Outlook mobile (iOS/Android) | 33 MB |
| Exchange Server (on-premises), default | 10 MB |
Two traps in that table. The first: Microsoft's own troubleshooting article confirms the 20 MB desktop limit is ignored for Exchange accounts, which use the server's limit instead — so the same file can succeed on one account and fail on another on the same PC. The second: your limit says nothing about the recipient's. A 100 MB message your tenant happily accepts still bounces at the other end.
Note the mobile row too. At 33 MB, Outlook on your phone will refuse files your desktop sends without complaint, which is why "it won't send from my phone" is often a size problem rather than a phone problem. All figures are from Microsoft's Exchange Online limits service description. Practical rule: anything over about 10 MB, send as a link.
Why did Outlook send fine this morning and stop this afternoon?
Because you may have hit a rate limit rather than a fault. This is the cause almost nobody checks, and it looks exactly like a broken mailbox.
Microsoft 365 applies two separate throttles to every mailbox:
- 30 messages per minute — the message rate limit.
- 10,000 recipients per day — the recipient rate limit, counted on a rolling 24-hour basis. A message to 50 people spends 50 of them.
Trip the daily one and sending stops until the 24-hour window rolls past the point you crossed it. Nothing is broken, nothing needs repairing, and every troubleshooting step above will fail to help. If you've just sent a newsletter, a client mailout, or a big invoice run and sending died shortly after, check this before you rebuild a profile.
This is also why mail-merge and bulk sending through a normal mailbox is a bad idea. If you're regularly sending to hundreds of recipients, that belongs on a proper bulk-email service rather than your day-to-day account, and it's worth setting up before it silently takes your mailbox offline mid-campaign.
Your printer stopped emailing scans, but Outlook still works. What changed?
This one catches small businesses out, and it isn't an Outlook problem at all. Microsoft is retiring basic authentication (plain username and password) for SMTP AUTH client submission in Exchange Online — the method scanners, multifunction printers, alarm systems, accounting packages and old line-of-business apps use to send mail through Microsoft 365.
The timeline has moved repeatedly. The current position is that Microsoft will disable basic authentication for SMTP AUTH at the end of December 2026, with administrators able to temporarily re-enable it and a final cut-off to be announced during 2027. Outlook on the desktop is unaffected — it connects over MAPI over HTTP with modern authentication, not SMTP AUTH.
If a device stops sending and there's no vendor firmware update that supports OAuth, the usual fix is to stop routing it through Microsoft 365 and point it at a dedicated SMTP relay instead. That's a printer and scanner setup job more than an email one, and it's worth sorting before the deadline rather than the morning a scan-to-email workflow dies. If you're running a small team on Microsoft 365, it's the sort of thing an ongoing IT support arrangement should be flagging for you in advance.
Does the new Outlook change any of this?
Not much, yet. New Outlook for Windows is still opt-in — off by default, with a Try the new Outlook toggle in the top right — and existing classic Outlook installs stay supported until at least 2029. You are not being forced across this year.
Two things worth knowing if you have flipped the toggle. The new app doesn't use the old profile system, so the Control Panel > Mail steps above don't apply to it. And if sending broke on the same day you switched, toggle back to classic before troubleshooting anything else — it takes seconds and immediately tells you whether the app or the account is at fault.
When is it worth calling someone in?
- You think the account has been compromised — messages sending that you didn't write, or unexpected password reset emails. Stop and get help with checking the machine and securing the account before changing settings; some fixes destroy the evidence of what happened.
- Outlook is crashing, not just failing to send — that points at a damaged data file, and DIY repair attempts on a damaged file are how mail gets permanently lost.
- The computer is slow or noisy as well — a failing drive corrupts Outlook's local data, and no amount of email troubleshooting fixes hardware. That's a repair job, not an email one.
- A new profile didn't help — that rules out most of what you can fix from the desk, and the remaining causes need access to the mail server or the account itself.
Frequently asked questions
Why is my email stuck in the Outbox?
An Outbox message means Outlook accepted it from you but hasn't handed it to the mail server. In order of likelihood: Work Offline is switched on, the attachment is over the size limit, you've hit a sending rate limit, or your saved sign-in has expired. Press F9 and read the status bar at the bottom of the window — it usually names the actual error.
How do I delete an email stuck in the Outbox?
Switch on Work Offline first (Send / Receive tab), then open the Outbox and delete the message or drag it into Drafts, then switch Work Offline back off. If you try to delete it while Outlook is online, Outlook may be part-way through sending it and will refuse, or send a duplicate.
Why does Outlook keep asking for my password?
Because the token Outlook uses to prove who you are has expired or been revoked — commonly after a password change, a new multi-factor setup, or a policy change at work. Signing in to Windows doesn't refresh it. Use File > Account Settings > Account Settings > Repair to re-authenticate, and if that loops, remove and re-add the account.
What's the maximum attachment size in Outlook?
20 MB for POP and IMAP accounts in Outlook desktop, 25 MB for Outlook.com, 150 MB for Microsoft 365 on desktop, 112 MB on the web, and 33 MB on Outlook mobile. Encoding inflates files by around a third in transit, so an 18 MB file can breach a 20 MB cap. Send anything over roughly 10 MB as a OneDrive or Drive link instead.
Is there a limit on how many emails Outlook can send per day?
On Microsoft 365, yes: 30 messages per minute and 10,000 recipients per day, counted on a rolling 24-hour basis. Cross either threshold and sending stops with no clear error until the window passes. It's a common and frequently misdiagnosed cause of Outlook suddenly refusing to send after a bulk mailout.
Can I still send emails while Outlook is in Work Offline mode?
No. Work Offline stops all sending and receiving — messages queue in the Outbox until you go back online. That's exactly why it's the first thing to check, and why it's the right mode to be in when you need to delete something out of the Outbox.
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