Why are my Outlook emails not sending?

The two-minute test

1. On any device, go to outlook.office.com and sign in with your work email. If sign-in itself fails or loops, skip to cause 3.
2. Send a short email to your own personal address (Gmail, whatever you have) and one to a colleague.
3. Both arrive: the account works. The fault is in the Outlook app or the computer (causes 1, 2 and 9).
4. Neither arrives, or a bounce comes back within a minute: it's the account or the sending (causes 3 to 8). Read the bounce; it usually names the reason.

That saves the most common wasted hour: reinstalling Outlook to fix an account Microsoft had blocked.

The nine causes

1. Something is stuck in the Outbox

Looks like: the Outbox folder shows a number, and every new email queues behind it. Nine times out of ten the first one has a big attachment. Microsoft 365's default limit is 35 MB per message, Outlook on the web allows a bit less, and plenty of recipients accept less again.

Fix: open the Outbox, open the stuck message, remove the attachment, and send a link instead (Outlook offers "Upload and share" for OneDrive). If the message won't open or delete, turn on Work Offline (Send/Receive tab), delete it, then turn Work Offline off. Anything else in the Outbox usually goes on its own after that.

2. Work Offline, or no connection

Looks like: "Working Offline" or "Disconnected" in the bar at the bottom of classic Outlook. Nothing sends or arrives, but you can still read old mail. It gets switched on by accident more than you'd think.

Fix: Send/Receive tab → Work Offline to toggle it off. If it says "Trying to connect", check the Wi-Fi, and if you're on a VPN, try without it. The new Outlook app doesn't have a Work Offline button; if it shows as offline, the connection is the problem.

3. Your password changed, or an MFA prompt is waiting

Looks like: "Need password" at the bottom of Outlook, a sign-in box that pops up and disappears, or nothing at all because the sign-in window is hiding behind other windows. Common right after an admin resets a password, or the day MFA is turned on.

Fix: click "Need password" and complete the sign-in, including the number-matching prompt on your phone. If it keeps looping, sign out of Office entirely (File → Office Account → Sign out), close Outlook, open it and sign in again. If MFA was just switched on for your business, our MFA guide explains what the prompts mean.

4. The mailbox is full

Looks like: a warning that your mailbox is almost full, then sending stops, then receiving stops. Business plan mailboxes hold 50 GB; fifteen years of attachments gets there.

Fix: empty Deleted Items and Junk (space is only freed once mail is gone from there), then in Outlook on the web go to Settings → Storage to see which folders are big and clear out old attachments. Ask your admin to turn on the archive mailbox that comes with the Business plans so old mail moves out automatically.

5. You've hit a sending limit

Looks like: a mail merge, a newsletter sent from Outlook, or a big BCC list, and then a bounce about recipient limits, or Outlook refusing to send anything for the rest of the day.

Microsoft 365 allows 10,000 recipients per mailbox in a rolling 24 hours, 500 recipients per message by default (an admin can set up to 1,000), and 30 messages a minute. Distribution groups in your address book count as one recipient; names pasted into BCC count individually.

Fix: wait; the 24-hour window rolls on from when you sent. Then stop sending newsletters from Outlook. A proper email platform handles the volume, the unsubscribe link the law requires, and the authentication that keeps you out of junk. Splitting a list into batches from Outlook only gets you to cause 6 faster.

6. Microsoft has blocked the mailbox for suspected spam

Looks like: you can receive but not send, and every attempt bounces with "550 5.1.8 Access denied, bad outbound sender". Almost always this means someone got the password, sent thousands of emails overnight, and Microsoft shut the mailbox down. The block is doing its job.

Fix: this one needs an admin, and the order matters. First secure the account: reset the password, sign out all sessions, delete any inbox rules or forwarding you didn't create, confirm MFA is on. Then unblock it in the Microsoft Defender portal → Email & collaboration → Review → Restricted entities. Microsoft says the restriction usually lifts within an hour, occasionally up to 24. Unblocking without securing the account first just gets it blocked again, and tells you nothing about what the intruder read.

7. Bounces that look like "not sending"

Outlook did send it. A bounce (a non-delivery report, or NDR) came straight back, often from "postmaster" or "Mail Delivery Subsystem", and it feels like the email never went. The useful part is the code near the top:

• 5.1.1 or "user unknown": the address doesn't exist. A typo, or the person has left that company.
• 5.2.2 "mailbox full": their problem, not yours. Phone them.
• 5.7.1 and the rest of the 5.7.x family: the receiving server refused your message on policy grounds. If it's one recipient, you may be on their block list. If it's every external recipient since a website or DNS change, your SPF, DKIM or DMARC records are the cause; our guide to emails going to junk walks through fixing them.
• Anything starting with 4: temporary. The server is retrying, usually for a day or two, and you'll get a final answer either way.

Fix: read the code, then act on it. Microsoft's NDR reference (in the sources) explains each one and says which side the fault is on.

8. An app, scanner or website sending as you

Looks like: Outlook is fine, but scan-to-email on the printer, the website's contact form or the job software's invoices have stopped going out. What broke is a device signing in with a username and password over SMTP, which Microsoft has been steadily switching off. Turning on MFA, blocking legacy sign-ins or disabling SMTP AUTH all have the same effect, and the old "app password" workaround is gone.

Fix: each sender needs a supported route. Scanners usually go through a relay connector tied to your office's internet connection, or "direct send" if scans only go to people in the business. Website forms should use a transactional email service with its own SPF and DKIM records, not your mailbox. Job software usually sends from its own servers. As a last resort an admin can enable authenticated SMTP for one single-purpose mailbox in the Exchange admin center, and treat it as the weak spot it is.

9. A broken Outlook profile or add-in

Looks like: only this one computer. Outlook hangs on Send or crashes on start, while Outlook on the web works. A misbehaving add-in (a signature tool, a CRM plug-in, an old antivirus hook) is the usual culprit, then a corrupted profile.

Fix, in order: start Outlook in safe mode (Windows key + R, type outlook.exe /safe). If it sends, an add-in is the cause: File → Options → Add-ins → Manage COM Add-ins → Go, untick them all, then re-enable one at a time. If safe mode doesn't help, repair Office (Windows Settings → Apps → Microsoft 365 → Modify → Online Repair). Last resort, a new profile: Control Panel → Mail → Show Profiles → Add. Your mail syncs back down from the server, so don't delete the old profile until the new one is working.

Which one have you got?

The limits above are Microsoft's published defaults at 31 October 2026; an admin can change some of them.

Where we fit

Most of these are a same-day remote fix, and the ones that aren't (a blocked mailbox, a scanner that needs a relay, DNS records) are exactly what a managed IT support plan exists for. If Outlook has stopped and you don't want to work through nine causes yourself, raise it in the portal or call, tell us what the bounce says, and we'll take it from there.