Fixing Mail Header Issues: How to Identify and Resolve Invalid Character Errors

Mail headers are an essential part of any email communication, as they provide information about the sender, recipient, and other details of the email. However, sometimes mail headers may contain invalid characters, leading to issues in the email delivery process. In this guide, we will discuss how to identify and resolve invalid character errors in mail headers.

Table of Contents

  1. Understanding Mail Headers
  2. Identifying Invalid Characters in Mail Headers
  3. Resolving Invalid Character Errors
  4. FAQs

Understanding Mail Headers

Before diving into the solutions, it is essential to understand what mail headers are and how they work. Mail headers are a series of lines in an email that provide information about the email's origin, destination, and other metadata. Some common mail header fields include:

  • From: Specifies the sender's email address
  • To: Specifies the recipient's email address
  • Subject: Provides the email subject
  • Date: Indicates the date and time the email was sent
  • Content-Type: Specifies the email content format (e.g., text/plain, text/html)

For more information on mail headers and their format, you can refer to the RFC 5322 standard.

Identifying Invalid Characters in Mail Headers

Invalid characters in mail headers can cause various issues, such as email delivery failures or incorrect display of email content. The most common invalid characters are non-ASCII characters or control characters.

To identify invalid characters in mail headers, you can use the following methods:

  1. Manual inspection: Open the email source (usually available in the email client's "View Source" or "Show Original" option) and examine the mail headers for any unusual or non-ASCII characters.
  2. Regular expressions: Use regular expressions to search for non-ASCII or control characters in the mail headers. For example, you can use the following regex to find non-ASCII characters: /[^\x00-\x7F]/. Similarly, you can use /[\x00-\x1F\x7F]/ to find control characters.

Resolving Invalid Character Errors

Once you have identified the invalid characters in the mail headers, you can follow these steps to resolve the errors:

  1. Remove or replace invalid characters: Remove the invalid characters from the mail headers or replace them with valid ASCII characters. Ensure that you don't change the header field's semantics while making these modifications.
  2. Use MIME encoding for non-ASCII characters: If you need to use non-ASCII characters in mail headers, consider using MIME encoding. This encoding allows you to use non-ASCII characters in mail headers without causing any issues. For example, you can encode a non-ASCII subject as follows: Subject: =?UTF-8?B?U29tZSBub24tQVNDSUkgY2hhcmFjdGVycw==?=.
  3. Verify email generation code: If you are using a programming language or library to generate emails, ensure that the code or library is correctly handling mail headers and not introducing invalid characters. Check the documentation and ensure that you are using the appropriate functions or methods to set mail header fields.


1. What are the valid characters in mail headers?

According to the RFC 5322 standard, mail headers can contain printable ASCII characters (excluding control characters) and some whitespace characters (space and horizontal tab). Non-ASCII characters and control characters are not allowed in mail headers unless properly encoded using MIME encoding.

2. Why do invalid characters in mail headers cause email delivery issues?

Invalid characters in mail headers can cause email delivery issues because they may not be correctly interpreted by email clients or mail transfer agents (MTAs). This can lead to mail being rejected, marked as spam, or misinterpreted by the recipients.

3. How do I check if my email client is correctly handling mail headers?

To check if your email client is correctly handling mail headers, send a test email with various header fields and examine the received email's headers. You can use tools like mail-tester to analyze your emails for any issues related to mail headers or email deliverability.

4. Can I use Unicode characters in mail headers?

You can use Unicode characters in mail headers, but they must be properly encoded using MIME encoding. This ensures that the characters are represented as ASCII characters in the mail headers and do not cause any issues during email delivery.

5. Are there any tools available to test and validate mail headers?

There are several tools available to test and validate mail headers, such as mail-tester, MX Toolbox, and Google's Message Header Analyzer. These tools can help you identify issues related to mail headers and provide suggestions for resolving them.

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