Why is MIME encoding headers in emails invalid and what are the consequences?
Summary
What email marketers say9Marketer opinions
Email marketer from Super User forum explains that improper handling of character encoding in email headers can lead to unreadable characters or mangled text in the recipient's inbox, diminishing the message's impact.
Email marketer from Reddit explains that improperly encoded headers can trigger spam filters, damage sender reputation, and result in poor email deliverability.
Email marketer from SendGrid explains that maintaining consistent and correct MIME encoding throughout the email, including headers, is essential for ensuring proper rendering and avoiding spam filters. Inconsistent encoding creates a negative signal.
Email marketer from Litmus shares that incorrect MIME encoding can cause emails to render differently across various email clients, leading to a broken or unprofessional appearance. This can negatively impact brand perception and engagement.
Email marketer from Email on Acid explains that incorrect MIME encoding in email subject lines, a type of header, can lead to emails being flagged as spam or displaying incorrectly. Proper encoding ensures readability and deliverability.
Email marketer from Gmass explains that diagnosing and fixing MIME encoding issues in headers can be complex, often requiring deep understanding of email standards. Failure to address these issues can severely impact campaign performance.
Email marketer from Mailjet explains that using invalid or non-standard encoding in email headers can significantly harm email deliverability. Many spam filters are triggered by these inconsistencies.
Email marketer from Stack Overflow shares that encoding the subject line is necessary when it contains non-ASCII characters to ensure it displays correctly across different email clients. However, over-encoding or incorrect encoding can lead to display issues.
Email marketer from Email Geeks shares that encoding headers means they likely won't work or be recognized, potentially failing to meet "Yahoogle" requirements.
What the experts say3Expert opinions
Expert from Word to the Wise explains that Gmail's 'Show Original' feature can be misleading because it might display decoded header content, even if the actual header is improperly MIME encoded. This can lead developers to believe the encoding is correct when it's not, and only looks right because Gmail is trying to be helpful.
Expert from Spam Resource explains that different email systems parse headers in various ways. Improper MIME encoding in headers might be tolerated by some systems but rejected by others, leading to inconsistent email delivery and display. This can result in unexpected filtering or rendering issues.
Expert from Email Geeks explains that encoding headers is invalid according to RFC standards, and relying on mailbox providers to accept them due to sloppy software development is not advisable. He suggests checking delivery metrics to assess the impact and considering it a bug to be fixed when possible. He also added if they are randomly MIME encoding headers then they’re Really Bad At Email, and I’d look at the other things they do.
What the documentation says5Technical articles
Documentation from RFC 1342 details how MIME encoding should be used for non-ASCII text in internet message headers, emphasizing the need for encoding only when necessary and using registered character sets. Incorrect implementation or unnecessary encoding breaks the standard.
Documentation from Oracle Communications Messaging Server explains that while some systems might tolerate MIME encoding in headers, it's generally considered incorrect. This is because headers are meant to be in ASCII or use specific encoding schemes designed for headers, as defined by RFC standards.
Documentation from RFC 5322 explains that header fields are defined as lines of ASCII characters, and to represent characters outside of ASCII, MIME encoding is used in the message body, not in the headers themselves. Encoding headers directly violates the standard.
Documentation from Microsoft explains that email messages must adhere to specific formatting rules, including correct header encoding. Deviations can lead to delivery failures and misinterpretation by email clients.
Documentation from MDN Web Docs explains that the Content-Type header should correctly describe the format of the message body. Mismatching or misrepresenting the content type can lead to rendering issues and security vulnerabilities. This is a type of MIME header.