How to Fix “This is not a valid file name” Error in Outlook when Sending Email?

When you send an email using the Microsoft Outlook client, the email isn’t sent immediately if you are using an Exchange, Outlook.com, or Microsoft 365 email account. The email you send or any changes you make to your mailbox are stored in the local cache, i.e., the OST file. Outlook frequently synchronizes the changes stored in the OST file to and from the mailbox server, thereby keeping the local mailbox and server mailbox updated and in sync.

If you try to send an email with attachments or files having invalid names, the message isn’t stored or sent. Instead, you encounter the following or similar error message in Outlook:

This is not a valid file name.
Try one or more of the following:
*Check the path to make sure it was typed correctly.
*Select a file from the list of files and folders.

The error prevents you from sending the email.

Below, you will learn various solutions to address the error and successfully send your emails without encountering the error in Microsoft Outlook.

Solutions to Prevent and Fix ‘This is not a valid file name’ Error in Outlook

Below are a few solutions that you can apply to resolve the error in Outlook 2010, 2013, 2016, 2019, and 2021.

Rename the File

This error may occur when you try to save a file from an Outlook email or attach the file with an invalid name or character in the file name. Make sure the file name does not contain the following characters:

• < less-than sign
• > greater-than sign
• / forward slash
• \ backward slash
• : colon
• | pipe

If your file name contains any of these characters, remove them and then try to save the file or share it via email.

Verify the File is Located at the Path

Sometimes, the file you inserted or attached to the mail you want to send may be wrong or invalid. It’s also possible that the file has been moved or has permission issues. This may result in the following error:

Open File Explorer and go to the location where the file is stored. Verify that the file is present, accessible, and isn’t locked. Then try to insert the file again and send the mail.   

Tip: You can also try renaming the folder containing the file, shortening the file name, and/or moving the file to a shorter folder path.

Use MSRA Tool

Microsoft Support and Recovery Assistant (MSRA) is a recommended tool to troubleshoot issues with Microsoft Office applications, such as MS Outlook. The MSRA runs a few tests on your system to detect what’s wrong and provides the best solutions to fix the issue.

To get started, download the Microsoft Support and Recovery Assistant setup and follow these steps:

• Install and launch the MSRA.
• Click ‘I agree.’
• Choose Outlook and click Next.
• Choose Other problems and click Next.
• Click Yes and then click Next.
• Continue to follow the wizard and use various available options in the MSRA to troubleshoot and fix the issues

Disable Add-Ins

Faulty or incompatible add-ins may prevent Outlook from working or lead to errors. Follow these steps to find and remove the faulty add-ins and fix the Outlook error.

• Press Windows + R keys, type Outlook /safe, and click OK or press the Enter key to open Outlook in safe mode.

• Go to File > Options.

• Click Add-Ins > Go.

Uncheck all add-ins to disable them and then click OK.

If disabling the add-ins resolves the error, open Outlook normally and activate one add-in at a time. Restart Outlook after enabling every add-in. This will help you find the faulty add-in as the error may pop up as soon as the specific add-in is enabled. Remove the add-in to solve the error permanently.

Recreate offline Outlook Data File

Sometimes, issues or inconsistencies in the Outlook data file (OST) can also lead to errors, such as ‘This is not a valid file name.’ To fix this, you can rebuild the OST file by following these steps:

• In Outlook 2010, 2013, 2016, 2019, or 2021, open Control Panel and go to User Accounts > Mail.
• Click on the ‘Data File…’ and select the email account where you are encountering the error.

• Click Open file location. This opens a File Explorer window showing the default OST file location.
• Copy the OST file to a safe location and then delete the file from the default location.  
• Start Microsoft Outlook.

To Read more, Check this blog too find the location of the OST file?

As soon as Outlook starts, it begins downloading and synchronizing the mailbox from the server to the new OST file.

Conclusion

Above we’ve listed the top methods to troubleshoot and fix the error ‘This is not a valid file name’ while sending a new email in Outlook. The error is usually caused due to special characters in the file name you’re sending. It may also pop up while saving a file from an email you received.

If nothing works, you can recreate the OST file to fix the issue. However, recreating OST can lead to missing mail items from your mailbox or Outlook. If you can’t find your emails or mail folders after recreating the OST, you can convert the backup OST file into PST using an OST to PST converter software, such as Stellar Converter for OST.

You can then import the converted PST into your email account using the Outlook Import Export wizard. The tool can convert inaccessible or orphaned OST file into PST, or export it directly to Microsoft 365 and Exchange Server.

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About The Author

Eric Simson

Eric Simson is an Email Platform Consultant and is associated with Stellar Data Recovery from last 6 years. He writes about the latest technology tips and provides custom solutions related to MS Outlook, MS Exchange Server, Office 365, and many other Email Clients & Servers.