Outlook Fix - Classic Outlook (Windows)
Fix Outlook error 0x800ccc78 (and 0x800ccc79)
Both of these errors mean the same thing in different words: your outgoing (SMTP) server refused to send the message. Error 0x800ccc78 means outgoing authentication is switched off, so the server does not recognise you as the sender. Error 0x800ccc79 is "relay not allowed" (a 550 rejection) - the server will not send on your behalf until you prove who you are. The fix for both is the same: tell Outlook the outgoing server needs a login, and use the modern submission port 587. Here is exactly how, or we will do it remotely in minutes. Rated 4.9 on Google.
- ● SMTP auth ON
- ● Port 587 STARTTLS
- ● Same-day remote fix
What it means
What 0x800ccc78 and 0x800ccc79 mean
These are a matched pair, and they are about sending, not receiving - your incoming email may work fine while messages pile up in the Outbox.
- 0x800ccc78 - Outlook tried to send but the outgoing (SMTP) server rejected the sender. Almost always this means "My outgoing server (SMTP) requires authentication" is switched off, so the server has no idea you are a legitimate account holder.
- 0x800ccc79 - "relay not allowed", a 550 rejection. The server refuses to relay (forward) your message onward because you have not authenticated. Some providers also require you to check mail first, then send.
The trigger is usually a password change that Outlook did not pick up, a provider switching off unauthenticated sending for security, or an ISP blocking the old port 25. The fix ties both errors together: switch on outgoing authentication and use the correct submission port.
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Step by step
How to fix it in classic Outlook
You will make two changes in the same place - turn on outgoing authentication, and set the outgoing port to 587.
- Open your account settings. Go to File > Account Settings > Account Settings, select your email account and click Change, then More Settings.
- Turn on outgoing authentication. Click the Outgoing Server tab and tick "My outgoing server (SMTP) requires authentication". Leave "Use same settings as my incoming mail server" selected - this is what fixes 0x800ccc78 and 0x800ccc79 in the vast majority of cases.
- Set the outgoing port to 587. Click the Advanced tab. Set the Outgoing server (SMTP) port to 587 and set encryption to STARTTLS (in older versions choose Auto or TLS). Port 587 is the modern authenticated submission port; the old port 25 is widely blocked by ISPs.
- Re-enter your password if it changed. If you recently changed your email password, Outlook may still be using the old one. Click OK, then on the account screen click Change and retype the current password so the outgoing login matches.
- Test it. Click Test Account Settings. The "Send test e-mail message" task should now show a green tick. Then send yourself a real email to confirm the Outbox clears.
Your username should normally be your full email address, not just the bit before the @. If sending still fails, your provider may need a specific outgoing server name or app-specific password - that is where we can step in.
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// GOOD QUESTIONS
Frequently asked
What is the difference between 0x800ccc78 and 0x800ccc79?
They are two sides of the same problem. 0x800ccc78 means the outgoing server rejected you because SMTP authentication is off. 0x800ccc79 is 'relay not allowed' - the server refuses to forward your message until you authenticate. Turning on 'My outgoing server (SMTP) requires authentication' fixes both.
Why can I receive email but not send it?
Receiving and sending use different servers. Your incoming server is working, but your outgoing (SMTP) server is rejecting the send because it cannot confirm you are a genuine account holder. Switching on outgoing authentication proves who you are and lets the message through.
Which port should the outgoing (SMTP) server use?
Use port 587 with STARTTLS encryption - the modern authenticated submission port. The old port 25 is blocked by most home and business ISPs to prevent spam, which is a common reason sending suddenly stops working.
I changed my email password and now sending fails - is that related?
Very likely. Outlook stores your outgoing password separately, and if it still holds the old one the server rejects the send. Open the account, click Change and retype your current password so both incoming and outgoing logins are up to date.
What should my SMTP username be?
For almost all providers it should be your full email address (for example you@yourdomain.co.uk), not just the part before the @. Using a shortened username is a frequent cause of authentication being refused.
Can 365 Techies fix the sending error for me?
Yes. We connect with your permission using Splashtop SOS - you watch throughout, we phone you first, and access ends the moment your email is sending again. Most outgoing-server errors are fixed within minutes. Call 01202 775566.
Emails stuck in the Outbox?
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