Few things are more frustrating than pressing Send and watching your message sit stubbornly in the Outbox. If you are dealing with an outlook smtp error, the problem almost always lies in your outgoing mail settings rather than in your inbox or the message itself. The good news is that SMTP problems are predictable, easy to diagnose, and usually fixed in a few minutes once you know where to look.
SMTP — Simple Mail Transfer Protocol — is the standard Outlook uses to hand your outgoing messages to your mail provider for delivery. When any detail of that handover is wrong — the server name, the port, the encryption type, or your login — Outlook cannot send, and it reports the failure as an SMTP error. In this complete guide, you will learn what an outlook smtp error means, the correct SMTP settings for every major provider, and a step-by-step sequence of fixes that resolves the overwhelming majority of sending failures.
Work through the checks in order. Most outlook smtp error cases are solved by the first three or four steps, and you can verify each change by sending yourself a test message before moving on.
What Is an Outlook SMTP Error?
An SMTP error in Outlook is a failure in the outgoing half of your email account. Your account has two halves: incoming mail (IMAP or POP3), which downloads messages to your inbox, and outgoing mail (SMTP), which sends the messages you write. An outlook smtp error means the incoming half is working fine — you can still receive mail — but the outgoing half cannot connect or cannot log in.
Outlook usually surfaces the problem with a message in the Send/Receive progress window or a small pop-up. Common examples include:
- Notices such as “Send test e-mail message: Unable to send this item” or similar send-failure warnings.
- Error codes such as 0x80042108 (“Outlook cannot connect to your outgoing (SMTP) e-mail server”), or 0x800CCC0E and 0x800CCC79, which point at connection or relay problems.
- “The server rejected your login” or repeated password prompts when you try to send.
- Messages that linger in the Outbox for hours and eventually bounce back as undeliverable.
Because receiving still works, an outlook smtp error can be confusing — everything looks normal until you try to reply. Keeping that distinction in mind makes troubleshooting much faster: you only need to look at the outgoing settings, the network path to the SMTP server, and anything that could block or interfere with it.
Correct SMTP Settings for Outlook
Before changing anything at random, compare your account against the known-correct settings. The single most common cause of an outlook smtp error is a wrong server name, port, or encryption method — often after a provider changes its requirements or a setup wizard guesses incorrectly.
| Provider | Outgoing (SMTP) server | Port | Encryption |
|---|---|---|---|
| Outlook.com / Hotmail / Live | smtp-mail.outlook.com | 587 | STARTTLS |
| Gmail (added to Outlook) | smtp.gmail.com | 587 | STARTTLS |
| Yahoo Mail (added to Outlook) | smtp.mail.yahoo.com | 587 | STARTTLS |
| iCloud Mail | smtp.mail.me.com | 587 | STARTTLS |
| Your own domain / generic IMAP | mail.yourdomain.com (or your host’s SMTP server) | 587 | STARTTLS |
Port 587 with STARTTLS is the modern default and the first combination to try. Port 465 uses implicit SSL/TLS instead — some providers accept it as an alternative, and if 587 fails, 465 is worth testing. Port 25 is the old standard, but many internet providers block it entirely (more on that below), so treat it as a last resort.
For Gmail and Yahoo accounts added to Outlook, there is one extra step: both require an app-specific password or OAuth sign-in rather than your normal account password when you connect through a desktop app. If your outlook smtp error appears as a login rejection on a Gmail or Yahoo account, generate an app password in your Google or Yahoo account security settings and use that in Outlook instead. This single step fixes a surprising number of “my password is definitely right” cases.
To find these settings in classic Outlook on the desktop, go to File > Account Settings > Account Settings, select your account, and click Change, then More Settings > Outgoing Server and the Advanced tab. In the newer Outlook for Windows, open Settings (the gear icon) > Accounts and select the account to review its server details.
How to Fix an Outlook SMTP Error
Work through these eight fixes in order. After each change, send a test message to yourself — when it arrives, you are done, and there is no need to continue down the list.
1. Verify the Outgoing Server Address
Open your account settings and compare the outgoing server field character by character with the table above. Typos are remarkably common: “smtp-mali.outlook.com” or “stmp.gmail.com” will fail every time, and Outlook’s error message will not always make the typo obvious. If you use a custom domain, check your hosting provider’s documentation — many hosts use something like “mail.yourdomain.com” or a server hostname such as “smtp.yourhost.com”.
While you are there, confirm the incoming settings too. If the incoming server is wrong, you may have a different problem altogether, but a quick glance costs nothing and rules out a misconfigured account.
2. Use the Correct Port: 587, 465 or 25
The port tells Outlook which “door” to knock on at the SMTP server. If the port is wrong, the connection fails before any login is even attempted — the classic “cannot connect to your outgoing server” outlook smtp error. Set the outgoing port to 587 first. If your provider’s documentation specifies 465 with SSL, use that instead.
Change the port on the Advanced tab of your account settings (classic Outlook) or in the account’s server settings (new Outlook). After changing the port, restart Outlook and test again. If 587 and 465 both fail, do not keep guessing — move on to the encryption and authentication checks below, because a port problem is rarely a port problem in isolation.
3. Turn On SMTP Authentication
Almost every modern mail provider requires you to prove who you are before it relays your message. In Outlook’s account settings, under the Outgoing Server tab, make sure “My outgoing server (SMTP) requires authentication” is ticked, and select “Use same settings as my incoming mail server”. An outlook smtp error with a 5xx code or a “relay denied” message almost always means this box is unticked.
If your provider gave you separate outgoing credentials — some business hosts do — choose “Log on using” and enter those details instead. Double-check the username format as well: many servers want your full email address as the username, not just the part before the @ symbol.
4. Match the Encryption: SSL vs TLS vs STARTTLS
Encryption protects your login and your message on its way to the server, but the server and Outlook must agree on the method. The options you will see are None, SSL/TLS (implicit encryption on port 465), and STARTTLS (encryption upgraded on port 587). A mismatch here produces connection timeouts and cryptic outlook smtp error messages that give little away about the real cause.
The safe pairings are simple: port 587 with STARTTLS, or port 465 with SSL/TLS. Never combine port 25 with SSL, and do not leave encryption set to None unless your provider explicitly tells you to — unencrypted SMTP is blocked by most providers and exposes your password on the network. If you are unsure which to pick, choose 587 with STARTTLS; it is the combination the big providers recommend and the one most setup guides assume.
5. Check for ISP Port-25 Blocks
Many home broadband providers block outbound traffic on port 25 to cut down on spam. If your account is set to port 25 and nothing else looks wrong, this block is a likely culprit. The fix is simple: switch to port 587 with STARTTLS, which ISPs leave open because it requires authentication and is far harder for spammers to abuse.
You can confirm a block by testing from a different network — tether your laptop to your phone’s mobile data and try sending. If the same outlook smtp error vanishes on mobile data, your home ISP is blocking the port, and 587 is your permanent answer. Note that this is entirely separate from Outlook itself; no amount of reinstalling the app will fix an ISP-level block.
6. Check Your Firewall and Antivirus
Security software sometimes intercepts outgoing mail to scan it, and after an update it can start blocking Outlook’s SMTP traffic by mistake. If your outlook smtp error appeared suddenly — everything worked yesterday — think about what changed overnight: an antivirus update is a prime suspect.
Temporarily disable your antivirus’s email-scanning or outgoing-mail shield (not the whole antivirus) and test sending. If mail flows, re-enable the shield and add Outlook as an allowed application, or switch the scanner to a less aggressive mode. Do the same check in Windows Firewall: Outlook should be listed as an allowed app. Remember to turn protection back on afterwards — the goal is diagnosis, not running unprotected.
Outlook’s own cached data can also interfere with sending in rare cases. If you have tried everything else, it is worth learning how to clear the Outlook cache safely before rebuilding anything more drastic.
7. Update Outlook to the Latest Version
Microsoft regularly updates Outlook’s handling of modern authentication and TLS versions. An outdated Outlook may try to negotiate an old TLS version that your provider no longer accepts, producing an outlook smtp error that looks like a settings problem but is really a software-age problem.
In classic Outlook, go to File > Office Account > Update Options > Update Now. In the new Outlook for Windows or the Microsoft Store version, updates arrive through the Store or Windows Update. After updating, restart your computer — not just Outlook — so the new networking components load cleanly, then test sending again.
8. Test Sending from Webmail
If Outlook still refuses to send, log in to your provider’s webmail (outlook.com, gmail.com, or your host’s webmail) and try sending from there. This is the great divider in outlook smtp error troubleshooting:
- Webmail sends fine: the problem is in Outlook or your computer — settings, profile, firewall, or software. Keep working the steps above, or try a new Outlook profile.
- Webmail fails too: the problem is with your account or provider — a locked account, a full mailbox, unpaid hosting, or a provider outage. Contact your provider’s support with the exact error message.
This single test saves hours. There is no point rebuilding your Outlook profile if the provider’s own webmail cannot send either.
Outlook SMTP Error Symptoms: What to Check First
If you would rather diagnose by symptom than work the full list, use this quick-reference table. Find the behaviour you are seeing, check the setting named, and set the value shown.
| Symptom | Setting to check | Correct value |
|---|---|---|
| Messages stuck in the Outbox, no error shown | Work Offline mode; outgoing port | Online mode; port 587 with STARTTLS |
| “Cannot connect to your outgoing (SMTP) server” | SMTP server name and port | Correct server from the table above; 587 or 465 |
| Repeated password prompts when sending | SMTP authentication; app password | Authentication switched on; app-specific password for Gmail/Yahoo |
| “Relay access denied” or 5xx errors | “My outgoing server requires authentication” | Ticked; “use same settings as incoming” |
| Sending works on mobile data but not home Wi-Fi | ISP port blocking | Switch to port 587; port 25 is likely blocked |
| Sending broke right after an antivirus update | Email shield / firewall rules | Allow Outlook; disable outgoing-mail scanning |
One more possibility worth mentioning: some outlook smtp error cases turn out to be send-and-receive problems in disguise — for example, a stuck synchronisation that keeps retrying the Outbox. If your Outbox messages refuse to budge even with correct settings, our guide to Outlook send and receive errors walks through the deeper repair steps.
Frequently Asked Questions
What is the correct SMTP server for Outlook.com?
For Outlook.com, Hotmail, Live and MSN addresses, the outgoing server is smtp-mail.outlook.com on port 587 with STARTTLS encryption, and SMTP authentication must be switched on using your full email address as the username. These settings also apply to Microsoft 365 personal accounts. If you see an outlook smtp error with these exact settings in place, the issue is more likely authentication or a network block than the server details.
Should I use port 587 or 465 for SMTP in Outlook?
Use port 587 with STARTTLS as your first choice — it is the modern standard that every major provider supports. Port 465 with SSL/TLS is the older alternative and works with most providers, so it is a good second thing to try if 587 fails. Avoid port 25: it is frequently blocked by internet providers and offers no advantage. Whichever you choose, the encryption setting must match the port.
Why does Outlook keep asking for my password when I send email?
Repeated password prompts during sending point to an SMTP authentication problem, not a wrong password. First, tick “My outgoing server (SMTP) requires authentication” and choose “Use same settings as my incoming mail server”. For Gmail and Yahoo accounts, generate an app-specific password in your account security settings and enter that in Outlook instead of your normal password. If prompts persist across the whole account, see our guide to fixing Outlook pii email errors, which covers the profile-level repairs that stop credential loops.
Can my antivirus cause an SMTP error in Outlook?
Yes. Antivirus email shields and firewalls can block or intercept Outlook’s connection to the SMTP server, especially after a definitions or program update. Temporarily disable just the email-scanning component and test sending; if it works, add Outlook as an allowed application and re-enable protection. Never leave your antivirus fully disabled as a permanent fix.
Why can I receive emails but not send them?
Receiving and sending use completely separate settings — incoming mail uses IMAP or POP3, while outgoing mail uses SMTP. An outlook smtp error therefore leaves your inbox working normally while everything you try to send fails. Check only the outgoing side: server name, port, encryption, and authentication. If webmail also fails to send, the problem is with your provider or account rather than Outlook.
Do I need to reinstall Outlook to fix an SMTP error?
Almost never. Reinstalling is a last resort and rarely fixes an outlook smtp error, because the settings that cause it live in your mail profile, not in the program files. A faster equivalent is creating a fresh Outlook profile (Control Panel > Mail > Show Profiles > Add) and adding your account there with the correct SMTP settings. Only consider reinstalling if multiple profiles fail identically on the same machine.