Email Not Sending? Discover Effective Fixes for Common Issues in 2024

You've hit send. The email sat in Outlook's Outbox for a moment. Now it's still there, in italics, and nothing's moving. Or worse, the message bounced back with a string of error codes that mean nothing to you.

This guide walks through the actual reasons UK business email stops sending, in order of how often we see them on our helpdesk. Most fixes take under five minutes once you know where to look.

Step 1: Are you actually online?

It sounds patronising but it's the cause about a quarter of the time. Outlook will happily queue a message in the Outbox if your internet connection has dropped, and won't always shout about it.

Check the bottom-right of Outlook. If it says "Working Offline" or "Disconnected", that's your answer. Click Send/Receive → Work Offline to toggle back online. If it still doesn't connect:

• Open a browser and load any website. If that fails, the issue is your internet, not Outlook. Restart your router and laptop.
• If you're on Wi-Fi, try moving closer to the access point or plugging in via Ethernet.
• For business networks, check whether your colleagues are also offline, a wider network outage needs the IT team, not Outlook tinkering.

Step 2: The mailbox is full

Microsoft 365 mailboxes have a quota, usually 50 GB or 100 GB depending on your licence. Hit it and outgoing email stops working, not just incoming. Outlook usually shows a warning along the lines of "Your mailbox is over its size limit" but it's easy to dismiss.

Fix it by:

1. Going to File → Tools → Mailbox Cleanup → View Mailbox Size.
2. Sorting by largest items: View → View Settings → Sort by Size.
3. Deleting or archiving the worst offenders (PowerPoints, video attachments, anything over 10 MB).
4. Emptying Deleted Items. Outlook doesn't free the space until you do.

If the mailbox is genuinely full of legitimate mail, your IT team can either upgrade the licence or set up an archive mailbox. Don't keep deleting things you actually need.

Step 3: The recipient bounced it back

If you've sent the email and got a "Mail Delivery Failure" or "Undeliverable" reply, that's a bounce. Read the bounce text, it's usually clear once you ignore the technical noise:

• "550 No such user" or "recipient address rejected", wrong email address. Check spelling.
• "552 Message too large", your attachment is over the recipient's size limit. Use SharePoint, OneDrive or WeTransfer instead.
• "Quota exceeded", their mailbox is full. Not your problem to fix; ring them.
• "Spam detected" or "blocked by content filter", your mail server's reputation is suspect, or the message contents triggered a filter (links to unknown domains, password-protected zips, certain phrases).
• "DMARC / SPF / DKIM failure", your email authentication isn't set up properly. This needs your IT team. It's a cyber security hygiene problem and worth fixing properly because it also affects deliverability for every other email you send.

Step 4: Outlook is stuck

Sometimes Outlook itself is the problem. The classic symptoms: emails sit in the Outbox, the send/receive progress bar spins forever, or the app freezes when you click send.

Quick fixes, in order:

1. Close Outlook completely (including from the system tray) and reopen.
2. Send/Receive All Folders (F9) to force a sync.
3. Open one of the stuck Outbox messages and re-save it, this often nudges it loose.
4. Restart in Safe Mode: hold Ctrl while opening Outlook, click Yes. If sending works in Safe Mode, an add-in is the culprit. File → Options → Add-ins → Manage: COM Add-ins → Go, disable them one at a time.
5. Repair the OST file: close Outlook, find the .ost file (usually C:\Users\YOU\AppData\Local\Microsoft\Outlook\), rename it to .ost.old, reopen Outlook. It will rebuild the cache from the server.

If repair doesn't help, run the Microsoft Support and Recovery Assistant, it diagnoses Outlook and Microsoft 365 issues automatically and is much better than guessing.

Step 5: Your password changed (and Outlook didn't notice)

After a password change, usually after MFA enrolment or a password expiry, Outlook can quietly fail to authenticate. You'll see a credentials prompt that does nothing when you fill it in, or messages stuck sending.

Fix:

1. Close Outlook.
2. Control Panel → Credential Manager → Windows Credentials. Delete every entry that mentions Outlook, Microsoft, Office or your work email.
3. Reopen Outlook. It'll prompt for credentials properly this time.

If you've recently turned on multi-factor authentication, you may need an app password for older Outlook versions, or to update Outlook itself to a version that supports modern authentication.

Step 6: It's a bigger Microsoft 365 problem

If nobody in the office can send mail, or if mail is sending but recipients aren't receiving it, the problem isn't on your laptop.

Check the Microsoft 365 Service Health Dashboard. If there's a flag against Exchange Online, sit tight, Microsoft is on it. If the dashboard is green and your team can't send, your IT support should:

• Check mail flow rules in Exchange admin (a recently-added rule can block more than intended).
• Check connector health if you have a hybrid setup or third-party mail gateway like Mimecast.
• Check DNS records (MX, SPF, DKIM, DMARC) haven't been changed.
• Look at mail flow → message trace for the last hour to see where messages are stuck.

This is bread-and-butter IT support work, and it's why having a managed provider matters when email, the most critical piece of business infrastructure most companies have, stops working at 9am on a Monday.

Step 7: The attachment is too large

Microsoft 365's default attachment limit is 25 MB, and even that is too big for many recipient servers. If you're sending a video, a large PDF or a folder of photos, the message will either bounce or sit forever.

Better options:

• Right-click attachment → Upload to OneDrive and send the link. The recipient gets a real file, not a copy.
• Use SharePoint for documents your team is collaborating on.
• For external sharing, WeTransfer, Dropbox or your CRM's secure share feature.

Repeatedly hitting attachment limits is a sign your business should be using OneDrive and SharePoint properly, talk to your cloud support provider about getting that set up.

Step 8: Your domain or IP is blacklisted

This is rarer but it's the worst one. Your email address sends fine, the recipient's mail server says no, and the bounce mentions Spamhaus, Barracuda or another reputation list.

Symptoms:

• All your outbound email to certain organisations is silently failing.
• Recipients in different companies (especially using Microsoft 365 themselves) say "I haven't seen anything from you in days".
• A bounce message references "sender blocked", "poor reputation" or links to a blocklist site.

This is almost always caused by:

• A compromised mailbox in your tenant sending spam (usually after a phishing attack, the reason phishing simulation training is worth doing).
• Missing or broken SPF / DKIM / DMARC records so receiving servers can't verify you.
• A misconfigured marketing tool sending bulk mail without proper authentication.

You'll need IT support to investigate. They'll check the tenant for unauthorised activity, fix the DNS authentication records, and submit delisting requests to whichever blocklist flagged you.

When to ring your IT support

You should not be fighting Outlook for an hour. As a rule:

• Under 10 minutes of trying basic fixes is fine.
• Email broken for the whole team, or bouncing from real recipients, call IT support immediately. Every minute is potentially a missed quote, contract or customer.
• Recurring "stuck Outbox" issues suggest a deeper problem with your Outlook profile or your Microsoft 365 setup.

If you don't have a proper IT support partner, this is exactly the kind of moment when you wish you did. We help Birmingham and West Midlands businesses sort exactly these issues every day, see our Birmingham IT support page for what working with us actually looks like.

Quick reference: the fastest fixes

Most email-not-sending problems are minor. The few that aren't tend to be the ones that quietly cost businesses real money, a missed contract, a customer who thinks you ignored them, a deal that drifted away. Treat email as critical infrastructure, because it is.

If the problem is an email you wish you hadn']t sent at all, our guide on how to recall an email in Outlook covers what actually works in new and classic Outlook, and what to do when recall fails.