Join us to show up for other Firefox users 🦊. Earn fun badges and Mozilla swag vouchers! Find out more: https://mzl.la/askafox150

Windows 10 reached EOS (end of support) on October 14, 2025. If you are on Windows 10, see this article.

Søg i Support

Avoid support scams. We will never ask you to call or text a phone number or share personal information. Please report suspicious activity using the “Report Abuse” option.

Læs mere

One of your filters uses a custom header that contains an invalid character ....

  • 1 svar
  • 1 har dette problem
  • 182 visninger
  • Seneste svar af Magini
  • Løst

When I send an email the following error message is displayed:

"One of your filters uses a custom header that contains an invalid character, such as ':', a non-printable character, a non-ascii character, or an eight-bit ascii character. Please edit the msgFilterRules.dat file, which contains your filters, to remove invalid characters from your custom headers."

This is followed by a message saying that the email has been sent and saved despite the error.

The problem happened after transferring to Windows 10 (from Win 7)

I have deleted ALL msgFilterRules.dat files and ALL filters, but the message persist.

Any suggestions on what else to try?

Thanks, Ben

When I send an email the following error message is displayed: "One of your filters uses a custom header that contains an invalid character, such as ':', a non-printable character, a non-ascii character, or an eight-bit ascii character. Please edit the msgFilterRules.dat file, which contains your filters, to remove invalid characters from your custom headers." This is followed by a message saying that the email has been sent and saved despite the error. The problem happened after transferring to Windows 10 (from Win 7) I have deleted ALL msgFilterRules.dat files and ALL filters, but the message persist. Any suggestions on what else to try? Thanks, Ben

Valgt løsning

Problem solved.

After deleting all imap mail folders/entries the problem went away.

Læs dette svar i sammenhæng 👍 0

Alle svar (1)

Valgt løsning

Problem solved.

After deleting all imap mail folders/entries the problem went away.