Summary
Invalid characters in email subject lines arise from several factors primarily related to character encoding. These include incorrect handling of character sets (especially non-ASCII characters), the use of unsupported Unicode characters, and issues with base64 encoding (like trailing zero bytes). The RFC 2047 standard dictates how non-ASCII characters should be encoded. Many recommend using UTF-8, ensuring the email client/server supports it, and testing across different platforms. Some servers only support 7-bit ASCII, necessitating MIME headers. Copy-pasting characters from external sources may introduce encoding issues. Properly encoding special characters/symbols and ensuring character set compatibility are essential for avoiding display problems.
Key findings
- Encoding Problems: Incorrect character encoding is a primary cause of invalid characters.
- UTF-8 Importance: UTF-8 encoding is crucial for broad email client compatibility.
- Non-ASCII Issues: Non-ASCII characters need proper encoding; otherwise, display issues arise.
- Base64 Anomalies: Base64 encoded subjects may have trailing zero bytes, leading to errors.
- Client Support: Some email clients might not support certain Unicode characters.
- 7-bit ASCII Restriction: Some email servers only support 7-bit ASCII, requiring MIME headers.
- RFC 2047 Compliance: Compliance to RFC 2047 is essential when transmitting Non-ASCII chars
Key considerations
- Verify Encoding: Double-check the character encoding of your email subject lines.
- UTF-8 Adoption: Use UTF-8 character sets for optimal results.
- Client-Server Support: Verify client/server support for UTF-8.
- Extensive Testing: Test subject lines across various email clients/platforms.
- Recode Characters: Manually type or recode special characters that may be incorrectly encoded when copy-pasted.
- MIME Headers Usage: Utilize MIME headers for compatibility with 7-bit ASCII servers.
- Trailing Zero Byte: Check the base64 encoded string by decoding it and looking at the hexidecimal values to ensure that there is no trailing zero byte
- Fallback strategies: When using chars outside of the Unicode Basic Multilingual Plane, it is important to test it and have fallback strategies.
What email marketers say
7 marketer opinions
Invalid characters in email subject lines are primarily caused by incorrect character encoding, particularly when using special characters or symbols. UTF-8 encoding is widely recommended for better compatibility across different email clients and platforms. It's important to verify the email client's encoding settings, ensure characters are properly encoded, and test across various platforms to avoid display issues.
Key opinions
- Encoding Issues: Incorrect character encoding is the main cause of invalid characters.
- UTF-8 Encoding: UTF-8 encoding is recommended for broad compatibility.
- Special Characters: Special characters and symbols need proper encoding.
- Copy-Pasting: Characters copied from external sources may not be properly encoded.
Key considerations
- Client Settings: Check email client and platform encoding settings.
- Testing: Test subject lines across different email clients and platforms.
- Manual Entry: Manually type or recode special characters for correct encoding.
- Unsupported Characters: Be aware of special characters not supported by some email clients.
- Contact Support: Contact support to investigate the issue
Marketer view
Email marketer from Mailjet explains that invalid characters in email subject lines often result from incorrect character encoding. They recommend using UTF-8 encoding for broader compatibility and suggest checking the email client or platform's encoding settings.
17 May 2025 - Mailjet
Marketer view
Email marketer from Campaign Monitor explains that using special characters not supported by some email clients, and double-check encoding settings to ensure compatibility across various platforms.
1 Nov 2021 - Campaign Monitor
What the experts say
4 expert opinions
Invalid characters in email subject lines can stem from several encoding-related issues. A trailing zero byte in the base64 encoded subject or incorrect handling of character sets, especially non-ASCII characters, can lead to display problems. UTF-8 encoding is recommended, along with testing across different email clients and servers to ensure proper rendering.
Key opinions
- Trailing Zero Byte: Base64 encoded subject lines may contain a trailing zero byte causing issues.
- Non-ASCII Characters: Use of non-ASCII characters without proper encoding results in display issues.
- Character Set Handling: Incorrect character set handling during email composition leads to invalid characters.
- Older Email Clients: Older email clients struggle with improperly encoded characters.
Key considerations
- UTF-8 Encoding: Use UTF-8 character sets to support a wide range of characters.
- Client/Server Verification: Verify that the email client and server support UTF-8.
- Test Rendering: Test email subject lines across different email clients and servers.
- Check Encoding: Examine the hexadecimal representation of the subject line for encoding errors.
Expert view
Expert from Word to the Wise explains that incorrect handling of character sets when composing emails can result in invalid characters. Laura Atkins recommends using UTF-8 character sets and verifying that the email client and server support it, to avoid display issues across different receiving systems.
23 Mar 2024 - Word to the Wise
Expert view
Expert from Email Geeks provides a command to decode and examine the subject line's hexadecimal representation, revealing a trailing zero byte.
8 Dec 2024 - Email Geeks
What the documentation says
5 technical articles
Invalid characters in email subject lines often arise from improper handling of character encoding, especially when dealing with non-ASCII characters. RFC 2047 mandates proper encoding for these characters, often using 'encoded-words'. Incompatible character sets, unsupported Unicode ranges, and the lack of MIME headers for servers supporting only 7-bit ASCII can also contribute. Base64 encoding is useful for reliably transmitting arbitrary sequences of octets. Ensuring the correct character set and utilizing appropriate encoding schemes are crucial for proper delivery and display.
Key findings
- RFC 2047: Subject lines must be encoded according to RFC 2047 for non-ASCII characters.
- Incorrect Character Sets: Using incorrect character sets or code pages leads to invalid characters.
- Unsupported Unicode: Unsupported Unicode characters by the email client or system can cause issues.
- 7-bit ASCII: Certain email servers only support 7-bit ASCII requiring MIME headers for other characters.
- Base64 Encoding: Base64 is useful for encoding content to ensure reliable transmission.
Key considerations
- Encoding Standards: Adhere to RFC 2047 for encoding non-ASCII characters.
- Character Set Selection: Ensure the correct character set is selected in the email client.
- Unicode Support: Use widely supported Unicode ranges or provide fallbacks for unsupported characters.
- MIME Headers: Use MIME headers for emails sent to servers that only support 7-bit ASCII.
- Base64 Implementation: Consider using base64 to avoid unreadable chars
Technical article
Documentation from Microsoft explains that incorrect character sets or code pages used when composing an email can lead to invalid characters. They advise ensuring the correct character set is selected in the email client or program being used.
19 Feb 2025 - Microsoft
Technical article
Documentation from ietf.org details the standards for representing non-ASCII text in email headers, stating that invalid characters may appear if the subject line isn't properly encoded according to RFC 2047. It highlights the importance of using 'encoded-words' to represent characters outside the ASCII range.
2 Aug 2024 - ietf.org
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