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Outlook error fix · Classic Outlook (Windows)

Fix Outlook Error 0x80042109

Outlook error 0x80042109 means "Outlook cannot connect to your outgoing (SMTP) server" — so your email receives fine but won't send. The fix is almost always the outgoing port and the outgoing-authentication tick, plus clearing anything stuck in your Outbox. Here's exactly what to change in classic Outlook.

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By the 365 Techies team · Reviewed 6 October 2026

What it means

What error 0x80042109 actually means

The full message is "Sending reported error (0x80042109): Outlook cannot connect to your outgoing (SMTP) e-mail server." The key word is outgoing — receiving usually works fine, it's only sending that fails, and messages pile up in your Outbox.

In classic Outlook this nearly always comes down to one of three things:

  • Outlook is trying to send on port 25, which most internet providers now block — it needs to be the port your provider lists, such as 587 with STARTTLS (Microsoft 365, Outlook.com) or 465 with SSL (BT, Virgin Media).
  • The "My outgoing server (SMTP) requires authentication" box is unticked, so the server rejects the message.
  • An oversized or corrupt message is jamming the Outbox, and every send attempt keeps failing on it.

// WINDOWS PCS ONLY

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The steps from this guide, one at a time, in the order that fixes the most cases fastest. Tell it what happened and it shows the next step. Nothing here leaves your device.Last reviewed 6 October 2026

Checking which Windows you have…STEP 1 OF 7
  1. 01Open Account Settings

    Go to File > Account Settings > Account Settings, select your account on the Email tab, click Change, then More Settings.

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Step by step

How to fix it in classic Outlook (Windows)

Do these in order. If Outlook keeps trying to send while you work, switch to Work Offline (Send/Receive tab) first so it stops re-triggering the error.

  1. Open your account settings. Go to File > Account Settings > Account Settings. On the Email tab, select your account and click Change, then More Settings.
  2. Turn on outgoing authentication. On the Outgoing Server tab, tick "My outgoing server (SMTP) requires authentication" and leave "Use same settings as my incoming mail server" selected. This alone often fixes it.
  3. Fix the outgoing port. On the Advanced tab, change the outgoing server (SMTP) port from 25 to 587 and set encryption to STARTTLS (BT and Virgin Media, among others, use 465 with SSL instead). Click OK.
  4. Re-enter your password. If your email password changed recently, click Change and retype it (use an app password if your provider, such as Gmail or Yahoo, requires two-step verification).
  5. Test the settings. Click Test Account Settings — the outgoing (SMTP) test should now pass with a green tick.
  6. Clear the stuck Outbox. If one large message is jamming everything, go to your Outbox, open the stuck message and either delete it or drag it to Drafts so it stops re-sending. (Keep attachments under about 20 MB.)
  7. Go back online and send. Turn off Work Offline and click Send/Receive. Your queued mail should now go out.

None of these steps touch your received mail — you're only correcting how Outlook sends and clearing the queue.

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// GOOD QUESTIONS

Frequently asked

Why can I receive email but not send it?

Error 0x80042109 is specifically an outgoing (SMTP) problem, so incoming mail is unaffected. It usually means Outlook is trying to send on the blocked port 25, or the 'My outgoing server (SMTP) requires authentication' box is unticked. Switch the outgoing port to the one your provider lists (587 with STARTTLS, or 465 with SSL for BT and Virgin Media) and tick outgoing authentication.

Which SMTP port should I use for sending?

In classic Outlook, set the outgoing (SMTP) port to 587 with STARTTLS. Most internet providers block the old port 25, which is why sending fails. Some providers, including BT and Virgin Media, use 465 with SSL instead, so check your provider's own mail settings.

My email is stuck in the Outbox — how do I clear it?

A single oversized or corrupt message can jam the whole queue. Switch Outlook to Work Offline (on the Send/Receive tab), open your Outbox, then delete the stuck message or drag it into Drafts so it stops re-sending. Keep attachments under about 20 MB, turn Work Offline back off and click Send/Receive.

Do I need to tick 'outgoing server requires authentication'?

Almost always, yes. Nearly every modern mail provider requires the sending server to verify who you are, so leaving this unticked makes the server reject your mail with 0x80042109. Tick it on the Outgoing Server tab and leave 'Use same settings as my incoming mail server' selected.

Could my firewall or antivirus block outgoing mail?

It can. An over-protective security suite or firewall can block Outlook's connection on the outgoing port. Temporarily disable email scanning to test; if sending then works, add Outlook as an exception and turn protection back on. A VPN can cause the same symptom — try disconnecting it.

Can 365 Techies fix 0x80042109 remotely?

Yes — it's a quick one for us. We connect securely with Splashtop SOS (you watch the session, we phone you first, and access ends the moment we finish), correct your outgoing-server settings, clear the stuck Outbox and test that email sends. Same day, usually within minutes. Call 01202 775566.

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We'll connect securely, correct your outgoing-server settings, clear the stuck Outbox and test it end-to-end — usually within minutes. You watch throughout, we phone first, and access ends when we're done.

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