You probably already know Outlook is the most widely used email app out there, hands down. It’s not just an email client, either. You get a calendar, task manager, contact manager, note-taking tool, journal, and more, all bundled in. But for all that popularity, Outlook is still pretty prone to errors.
Quick Navigation:
About Exchange Proxy Server: Why it is unable to connect?
Why Does Outlook Say Unable to Connect to Proxy Server?
How to fix the Proxy Server Security Certificate in Outlook?
About Outlook Exchange Proxy Server: Why it is unable to connect?
Exchange Server:
An Outlook exchange server relies on the Mailbox server role (a multi-role server) to send outbound emails through a send connector. The Send Connector is what lets you send emails from your Exchange server out to other domains on the Internet. If you click into the properties of Exchange’s send connector, you’ll find an option to proxy through a client access server.
A Proxy Server:
A proxy server is a network that your client’s queries or requests get routed through on their way to the main server. In short, it acts as a middleman network, passing requests back and forth between clients and the server. The “Outlook is unable to connect the proxy server” error you’re seeing is tied to a security certificate issue on that proxy server.
A Security Certificate:
A security certificate authenticates your MS Exchange server so it only communicates with servers it trusts. When Microsoft Outlook tries to connect to a server using a remote procedure call (RPC) connection or a secure HTTP (HTTPS) connection, you’ll see one of these error messages:
-
- There is a problem with the proxy server’s security certificate. The name on the security certificate is invalid or does not match the name of the target site. The outlook is unable to connect to this server (Error code 10).
-
- There is a problem with the proxy server’s security certificate. Outlook anywhere is unable to connect to the proxy server (Error code 10).
-
- There is a problem with the proxy server’s security certificate. The security certificate is not from a trusted certifying authority. The outlook is unable to connect to this server (Error code 10).
Let’s dig a little deeper and walk through why this error actually happens:
Why Does Outlook Say Unable to Connect To Proxy Server?
There are a handful of reasons you might run into the “Outlook cannot connect to the server” error:
Here are a few of the biggest culprits behind the “The outlook is unable to connect to the server” error:
- You’re trying to connect to a server, and that connection needs a certification authority.
- The certification authority at the root can’t be used.
- The certificate might be invalid or denied.
- The site name doesn’t match the name on the certificate.
- A third-party add-in, or a third-party browser add-in, is blocking access, or sometimes it comes down to corruption in the Outlook file.
Outlook unable to connect proxy server error code 10 and error code 20
The codes attached to this error are not random. They point at roughly where the connection broke down, and treating them as the same problem wastes time.
Error code 10 almost always means a DNS or routing failure between your machine and the proxy, not a certificate problem at all. Outlook is unable to connect to the proxy server error code 10 shows up a lot on networks with split DNS, where internal and external name resolution disagree about what the Exchange server’s address actually is. Flushing DNS (ipconfig /flushdns from an elevated command prompt) clears a surprising number of these.
Error code 20 sits closer to the certificate side. Outlook is unable to connect to the proxy server (error code 20) typically shows up when the certificate chain is incomplete, meaning the intermediate certificate is missing even though the root certificate installed fine. Checking the certificate chain under the Certification Path tab, mentioned earlier in this article, is the fastest way to confirm that before you go looking anywhere else. On Outlook 365 builds specifically, this error can also follow a tenant migration where the old Autodiscover record is still cached; running Outlook with the /resetautodiscover switch from the Run dialog forces it to look the record up fresh, and Outlook unable to connect proxy server 365 complaints often clear up at exactly that step.
Why does Outlook keep saying “Cannot connect to server”?
Nine times out of ten it is not actually your internet connection. Outlook throws this message when it cannot complete the handshake with Exchange, and that can happen for reasons that have nothing to do with the proxy certificate issue covered above. A dropped VPN, an expired password that Outlook has cached, or a firewall rule blocking port 443 will all produce some version of this message.
I have also seen it caused by Outlook running in offline mode by accident. Check the Send/Receive tab first and make sure Work Offline is not toggled on, since it is easy to click without noticing. If you are on a laptop that recently changed networks, for example moving from office Wi-Fi to a home router, Outlook sometimes holds onto the old network profile and needs a restart to pick up the new one.
If none of that fixes it and the same message keeps coming back on a schedule, the issue is usually server-side. That is a different problem from the one covered on our Outlook cannot connect to server troubleshooting page, which goes through the connectivity side step by step rather than the certificate side this article focuses on.
I keep getting a Microsoft proxy authentication error in Outlook. How can I fix this?
Proxy authentication error 2606 and similar codes mean Outlook reached the proxy but could not prove it was allowed through. That is different from the certificate trust problem described earlier in this article. Authentication errors usually trace back to cached credentials that no longer match what the server expects, especially after a password reset or an account migration to Microsoft 365.
Clear the stored credentials first. Open Control Panel, go to Credential Manager, and remove any entries listed under Windows Credentials that reference outlook.office365.com or your Exchange server name. Restart Outlook and let it prompt you for a fresh login. Do not reuse autofill here; type the password manually once, since a stale autofill entry is often the actual culprit.
If the prompt keeps rejecting a password you know is correct, check whether your organization has modern authentication (OAuth) enabled. Older Outlook builds configured for basic auth will fail silently against a tenant that has switched over, and the fix is a profile rebuild rather than another password attempt.
How Do I Fix the Proxy Server Security Certificate In Outlook?
Here are a few ways to fix the Outlook unable to connect to proxy server error:
Verify the certificate:
If you’re dealing with an invalid certificate name, or Outlook just won’t connect to the proxy server, there are two things worth checking closely: the certificate’s expiry date, and whether the name in the subject field matches the site’s name.
Follow the steps below to examine the certificate:
- Open Internet Explorer and connect to any secure, trustworthy server, then press Enter. Or type “https://www.[servername].com/rpc” into the browser’s address bar and hit Enter. Then connect with the RPC server.
- Once you spot the padlock icon in the lower-right corner of the browser window, double-click it and select the Details tab.
- Jot down what’s listed in the Valid to and Subject fields.
If the date checks out and the subject matches the site name, your certificate is up to date. If not, it’s either expired or the site name needs fixing.
Install the trusted certificate:
If you’ve gotten an error saying the security certificate isn’t from a trusted authority, or that it hasn’t been added to Microsoft Management Console (MMC, which holds all the certificate snap-ins), here’s what to do.
Following are the steps to install the trusted root certificate:
- When the Certificate dialogue box opens, click Install Certificate, then Next to move ahead.
- Check the box for Place all certificates in the following store.
- Now click Browse to pick Trusted Root Certification Authorities, then click OK. Click Next to keep going.
- Wait for it to finish. Once it’s done, click Finish.
If the error’s still hanging around and won’t go away, try adding the certificate snap-in to MMC instead.
- Open Run (Windows + R) and type mmc. Click Yes.
- Go to File > Add/Remove Snap-ins.
- Select Certificates, then choose Add.
- Click My user account.
- Select Add again, and this time choose Computer Account.
- Move the new certificate from Certificates-Current User > Trusted Root Certification Authorities over into Certificates (Local Computer) > Trusted Root Certification Authorities.
Turn off third-party add-ins:
Third-party add-ins are behind most Outlook errors, including the proxy server error, 0x80040154 and plenty of others. If you’re seeing the error about the security certificate being invalid or not matching the target site’s name, follow these steps:
- If Outlook is open on your system, close it, then press Win+R to open the Run window.
- In the Run search bar, type “outlook.exe /safe” and click OK.
- If safe mode runs fine with no issues, that tells you the problem is coming from a third-party add-in.
- To turn off third-party add-ins, go to the File menu at the top of the Outlook window.
- Select Options from the file menu, then click Add-ins.
- Find the Manage box and click COM Add-ins.
- Click Go to continue.
- See the checkbox next to the third-party add-in? Uncheck it.
- Restart Outlook.
If none of these fixes worked for you, the problem might actually be sitting in the Outlook file itself. Your Outlook PST or OST could be corrupted, maybe from malware, maybe for some other reason.
Outlook PST files often live on network servers. If your PST file is sitting on a server that’s having trouble, it might become inaccessible, and if the server goes down entirely, your PST file could end up corrupted too.
In situations like this, you can turn to the SFWare Outlook repair tool to fix any corrupt Outlook file. There’s no harm in working through every possible solution to get the app running again.
SFWare Outlook PST Repair tool is a specialized tool to fix any damage or corrupt Outlook data file. Built with features like Normal Scan and Smart Scan help in fixing the complex PST file issues easily. The tool works on read only mode, to ensure no harm is caused to generate a healthy PST file. Download the software now and fix the PST file corruption in few simple steps.
How do I set a proxy in Outlook?
Outlook itself does not have a standalone proxy field the way a browser does. It inherits proxy settings from Windows, so you set it at the system level, not inside Outlook’s own options.
- Open Settings, then Network & Internet, then Proxy.
- Under Manual proxy setup, turn on Use a proxy server and enter the address and port your network administrator gave you.
- Restart Outlook so it picks up the new connection path.
On older machines running Windows 7, the path is Control Panel, Internet Options, Connections tab, LAN settings, where the same proxy address and port fields sit under a slightly different name. This is where the Outlook unable to connect proxy server windows 7 cases usually trace back to, a leftover or mistyped proxy entry from a machine image that was never cleaned up.
If your Exchange connection goes through Outlook Anywhere rather than a general network proxy, the proxy settings live inside the Exchange account configuration instead, under More Settings, Connection tab, Exchange Proxy Settings. That is a separate setting from the Windows-level proxy and changing one does not touch the other, which is a common source of confusion when someone fixes the wrong one and the error persists.
Conclusion
As we’ve covered above, “Outlook is Not Able to connect to the proxy server” is an error tied to the exchange server. With the methods laid out here, you can work through fixing the error and get Outlook running smoothly again. And if your Outlook file turns out to be corrupted, you can always count on the SFWare Outlook repair tool to help you out.
Frequently Asked Questions:
1: How do I fix a Security certificate error in Outlook?
Start by verifying the certificate and installing the trusted root certificate. If the error is still there and won’t clear up, try adding the certificate snap-in to MMC. You can follow the detailed steps above to fix a Security certificate error in Outlook.
2: How do I restore my Certificates in Outlook?
Here’s how to restore the default SSL Certificate and Private Key: go to Control Panel >> System >> Security >> Certificate & Private Key. Click Restore to Default. A confirmation message will pop up >> click Ok.
3: How do I find my Certificates in Outlook?
Here’s how to view Certificates in Outlook. First, on the File tab >> click Options >> click Trust Center >> click Email Security. Go to Encrypted e-mail >> click Settings >> under Certificates and Algorithms >> click Choose. Now click on the certificate you want, then click View Certificate.
