Error 421 is a common SMTP (Simple Mail Transfer Protocol) Outlook error message. MS Outlook is one of the best and popular email applications widely used by PC Windows users for sending and receiving emails within and outside workplaces.
It is most commonly used in offices by professionals for faster communication.
This error may occur while sending and receiving emails on the server. The error 421 messages is displayed in either of the following formats:
“An unknown error has occurred: Server error 421”
"Could not send your message. Error 421"
421 4.2.1 : (DYN:T1) http://postmaster.info.aol.com/errors/421dynt1.html
421 4.3.1 Out of disk space for temp file
421 4.3.2 All server ports are busy
421 Downstream server error
421 Fork failed
421 Greylisting enabled, please try later
“An unknown error has occurred. Subject ‘Testing’, Account: ‘pop.charter.net’, Server: ‘mail.charter.net’, Protocol: SMTP, Server Response:
’421 Message rejected’, Port: 25, Secure(SSL): No, Server Error: 421, Error Number: 0x800CCC67”
Error CausesSome common causes for this critical Outlook error are:
This is a critical Outlook error that should be resolved timely. If you delay, it may cause you a great deal of inconvenience.
The error code 421 hampers your ability to access your email account, send and receive emails, add new contacts, view emails, calendar entries, and journals. In other words, it just brings your email activities to a standstill locking you out of the account.
To resolve this error code on your system, you don’t always have to hire a professional or call the network personnel to do the job. It may be a critical error but the good news is that it is quite easy to fix. Even if you don’t have technical expertise, you can still repair it.
Here are some of the best methods to repair this issue on your system:
Try changing your server settings. Here set the inbound connections limit at the maximum limit.
After the process is complete, try accessing your Outlook email account again; if it works, then that’s great the error 421 is resolved. However, if it doesn’t, then the problem could be related to viral infection.
If the cause of the error is PST files corruption, then not only will you be locked out of your email account, you are also at risk of losing all the data stored in your email account.
However, the best way to resolve the error and ensure full data recovery is to download a reliable PST repair tool. By running a PST repair tool on your system, you can resolve the error 421 right away. It recovers your data, repairs corrupt and damaged PST files.
If viral infection and Trojans are causing the error 421 to pop up on your system, then it is recommended to download and scan your PC with Restoro.
This is an advanced, intuitive, and user-friendly PC fixer. It is not just an antivirus but also a system optimizer and a registry cleaner.
Unlike common antivirus software that only removes viruses and simultaneously slows down the system’s speed, Restoro removes all types of malicious software on your PC including malware, spyware, Trojans, and viruses, and also boosts the speed of your PC significantly.
It is safe, easy to operate, and a quick fixer. It is compatible with all Windows versions.
Click here to download Restoro and resolve error 421 today!

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Error Causes
It is not a secret that Windows 11 is the most secure Windows ever created, well at least up until now. It is packed with advanced security features like S-mode, Secure Boot, and Trusted Platform Module (TMP 2.0). All these new security features that Microsoft has advertised made people question are enough for security or do you still need third-party Antivirus software.
In order to get an answer to this question, let us first take a detailed look at what each one of them does, and then we will give you our opinion on a given subject.
takeown /f FileNameAndPathNote: In the given command, “FileNameAndPath” represents the name and the path of the corrupted file. For instance, the corrupted file is named “wuapi.dll”, then you need to type “takeown /f C:/Windows/System32/wuapi.dll” Step 5: Next, you need to grant administrators full access & control to the corrupted file and you can do that via Command Prompt again. Step 6: In the elevated Command Prompt, type and enter the following command to execute it:
icacls FileNameAndPath /GRANT ADMINISTRATORS:FNote: We’re going to use the wuapi.dll again as an example. So the command should be like this: icacls “C:/Windows/System32/wuapi.dll /grant administrators: F”. Step 7: Now all that’s left to do is replace the corrupted system file with a good copy from a trusted source. Once you have a copy, open an elevated Command Prompt again and execute the following command:
copy NewFileLocation FileNameAndPathNote: In the command given above, the “NewFileLocation” is the location where you have saved the copy of the file. For instance, if you’ve saved it on your desktop, this is how it should look: “copy C:/Users/<username>/Desktop/wuapi.dll C:/windows/system32/wuapi.dll”
Code 16 is a type of Device Manager error code. Windows XP users are most likely to encounter it. This error pops on XP systems that are running legacy or not plug-and-play hardware.
Error code 16 is displayed in the following format:
‘Windows cannot identify all the resources this device uses. (Code 16)’
Error CausesError code 16 occurs when you use a peripheral device such as an external drive or a printer and that device is not properly or completely configured. Though the device may have been recognized before Windows fails to verify its configuration. Simply put, error code 16 appears when devices are partially configured.
In addition to this, another cause of error 16 is outdated or corrupt device drivers. Devices often fail to run successfully because of driver problems.
Device drivers are programs that provide instructions to control different devices attached to the computer system such as audio devices and printers to name a few.
Though error code 16 is not fatal like other PC error codes such as BSoD; however, it may hamper your ability to use certain devices installed on your system and lower your work productivity.
To avoid inconvenience, it is advisable to resolve the error immediately to avoid inconvenience.
PC error codes are often considered technical and difficult to resolve which is why many prefer to hire a professional for the repair job rather than doing it themselves.
Small repair jobs like fixing error code 16 by a professional computer programmer may cost you hundreds of dollars.
So, why incur so much cost when you can repair it easily by yourself even if you are not a technical whiz.
Error code 16 is easy to repair.
Here are some effective DIY methods that you can use for resolving error code 16 on your PC. These methods require no technical background, knowledge or expertise for that matter. Just simply follow the instructions and resolve the error code 16 issue.
Let’s get started…
Run setup from the disk that came with the hardware device, or you can simply copy the setup software file to your hard drive/flash drive. Refer to the vendor’s direction to set up.
If for any reason you’re unable to find the directions, simply uninstall the device. Then go to the start menu, Control Panel, and select ‘Add New Hardware’.
Now pick the appropriate hardware and follow the steps that the Wizard suggests. This is most likely to resolve the error code.
Nonetheless, if the error code still persists, then try method 2.
Another alternative method to resolve error code 16 is to allocate more PC resources to the device. To specify additional resources:
As previously stated above, device manager error codes like error code 16 may also occur due to driver problems. If this is the underlying reason for error code 16 on your system, then the best way to resolve it is to install DriverFIX.
DriverFIX is user-friendly, advanced, and feature-filled software deployed with an intelligent programming system, which detects all problematic device drivers automatically in seconds.
It matches these drivers with their latest versions and updates them smoothly without any hassle, resolving error code 16 right away.
And furthermore, once you install this software on your PC, you don’t have to worry about driver problems anymore typically because it updates drivers on a regular basis with compatible and new versions, ensuring that your computer functions properly. It is easy to install and is compatible with all Windows versions.
Click here to download DriverFIX to repair error code 16