Last Updated: February 4, 2026
ISSUE
When you start a new email, view an existing email, or reply to an email you may see the error message:
“This is not a valid style name.”
The error occurs when Outlook is rendering the signature. In addition, you may see the signature in the email now has a <DIV> tag inserted.
This issue occurs with some signatures and if Outlook is set to use the message format as Rich Text (RTF).
STATUS: WORKAROUND
Workaround #1
To work around the issue, set classic Outlook to use HTML for the message format.
-
Select File > Options > Mail.
-
Set Compose messages in this format: to HTML.
Workaround #2
To immediately stop the issue, disable the signature.
-
Select File > Options > Mail > Signatures.
-
At the bottom of the Signatures and Stationary dialog box, set New messages: and Replies/Forward: to None.
Additional Resources
More Resources
Ask the experts
Connect with experts, discuss the latest Outlook news and best practices, and read our blog.
Get help in the community
Ask a question and find solutions from Support Agents, MVPs, Engineers, and Outlook users.
Suggest a new feature
We love reading your suggestions and feedback! Share your thoughts. We're listening.
See Also
Fixes or workarounds for recent issues in classic Outlook for Windows
