Why is MIME encoding headers in emails invalid and what are the consequences?
Matthew Whittaker
Co-founder & CTO, Suped
Published 20 Apr 2025
Updated 18 Aug 2025
5 min read
Email headers are like the postal information on an envelope, containing crucial metadata that directs your message. They specify the sender, recipient, subject, and technical routing information, guiding how mail servers process and deliver emails. MIME (Multipurpose Internet Mail Extensions) standards were developed to extend the basic ASCII-only email format to support non-ASCII text, attachments, and rich HTML content. However, the application of MIME encoding has strict rules, especially concerning email headers.
When MIME encoding is applied incorrectly or to headers where it isn't permitted, it can lead to invalid email structures. This misconfiguration can have significant consequences for email deliverability, authentication, and recipient experience. Understanding why certain headers shouldn't be MIME encoded is critical for maintaining healthy email sending practices.
The standards and their purpose
The core of email header validity lies in the foundational RFCs, primarily RFC 2047. This RFC specifically defines how encoded-words can be used within certain message header fields to carry non-ASCII text. Its purpose is to allow for characters beyond the standard 7-bit ASCII set in fields like Subject, From, and To. However, this encoding is not universally applicable to all header fields.
Many email headers, particularly those used for technical processing, routing, and authentication, are expected to contain only ASCII characters. These headers are parsed by mail servers and spam filters based on precise specifications. Encoding them using MIME encoded-word syntax, where it's not explicitly allowed by the relevant RFC for that specific header field, renders them invalid.
For instance, a List-Unsubscribe header should contain direct URLs or email addresses, not MIME encoded segments. Here is an example of an invalid MIME encoded List-Unsubscribe header. This is a common issue with List-Unsubscribe headers:
Invalid MIME encoding in email headers can significantly hinder your email deliverability. Mailbox providers and spam filters strictly interpret email headers to determine message legitimacy and routing. When they encounter improperly encoded headers, they may struggle to parse the information correctly, leading to various negative outcomes.
One common consequence is that emails might be flagged as spam or outright rejected. Automated systems are designed to detect deviations from RFC standards, and invalid encoding can trigger these filters. This increases your spam rate and can lead to your domain or IP address being placed on a blacklist (or blocklist), impacting future sending reputation.
Another issue is that the intended functionality of the header might break. For example, an improperly encoded List-Unsubscribe header might prevent recipients from easily unsubscribing, leading to increased spam complaints. This is especially problematic given the increasing emphasis on one-click unsubscribe requirements from major providers.
Some mailbox providers, like Gmail and Outlook, might occasionally accidentally accept or misinterpret invalid headers due to their own parsing flexibilities. However, relying on this behavior is risky. Standards evolve, and what works today might cause rejections or spam-folder placement tomorrow.
Impact on email authentication
Email authentication protocols like SPF, DKIM, and DMARC rely heavily on the integrity and correct formatting of email headers. Invalid MIME encoding in these headers can lead to authentication failures, making your emails appear suspicious to receiving mail servers.
Specifically, DKIM (DomainKeys Identified Mail) uses a cryptographic signature that covers specific parts of the email, including certain headers. If headers are improperly encoded, their content might be altered during transit or misinterpreted by the receiving server, causing the DKIM signature to fail. A failed DKIM check, especially when combined with SPF or DMARC issues, significantly increases the likelihood of your emails landing in the spam folder or being rejected outright.
Authentication impact
Receiving servers perform rigorous checks to verify email authenticity. When headers are improperly formatted due to invalid MIME encoding, it introduces inconsistencies that can undermine these checks. This can lead to a DMARC failure, even if your SPF and DKIM records are otherwise correctly configured. This type of failure signals to mail servers that your email might be forged or tampered with, drastically reducing its chances of reaching the inbox.
User experience and brand perception
Beyond technical failures, invalid MIME encoding can directly impact the recipient's experience and, by extension, your brand's perception. When email headers are garbled, it can lead to confusion and a lack of trust among your audience. This can severely affect engagement metrics and brand reputation.
For example, if the Subject line, which is one of the headers where MIME encoding is acceptable for non-ASCII characters, is incorrectly encoded, it will appear as a string of random characters, making the email appear unprofessional or even malicious. This can lead recipients to ignore or mark your emails as spam, regardless of content quality.
Problem: invalid headers
Technical impact: Leads to parsing errors by email servers, often resulting in rejection or spam classification.
User experience: Garbled subject lines or sender names confuse recipients, reducing open rates.
Solution: valid headers
Compliance: Adheres to email RFCs, ensuring proper processing across all mail servers.
Enhanced security: Ensures authentication passes for SPF, DKIM, and DMARC, building trust with providers.
Improved trust: Presents a professional image, leading to higher engagement and a stronger brand.
Views from the trenches
Best practices
Always validate your email headers against relevant RFCs, especially for technical headers.
Use email testing tools to check how different mailbox providers interpret your headers.
Educate your email platform or ESP about RFC compliance if they are generating invalid headers.
Common pitfalls
MIME encoding headers where it's not explicitly allowed, e.g., List-Unsubscribe.
Assuming that if an email is delivered, its headers are fully compliant.
Overlooking warnings from email testing services about header validity.
Expert tips
Consider MIME encoding for subject lines and sender names only if non-ASCII characters are present.
Review your email sending platform's documentation for header generation practices.
Regularly monitor your deliverability metrics for any sudden drops or increased rejections.
Marketer view
Marketer from Email Geeks suggests removing MIME encoding from headers beyond the subject line, as it is generally not standard practice.
December 1, 2024 - Email Geeks
Expert view
Expert from Email Geeks says encoding headers is invalid according to RFCs, though some mailbox providers might accept them due to lax software, which is not reliable.
December 1, 2024 - Email Geeks
Key takeaways
Ensuring your email headers are correctly formatted and adhere to MIME standards (where applicable) is a foundational aspect of email deliverability. Invalid MIME encoding in headers can lead to a cascade of problems, from emails being filtered as spam or blocked (blacklisted) to authentication failures and a degraded recipient experience. Prioritizing correct header encoding is essential for maintaining a strong sender reputation and achieving consistent inbox placement. Regularly reviewing your email headers and promptly addressing any encoding issues will help you navigate the complexities of email sending and ensure your messages reach their intended audience effectively.
Gmail and 
Outlook, might occasionally accidentally accept or misinterpret invalid headers due to their own parsing flexibilities. However, relying on this behavior is risky. Standards evolve, and what works today might cause rejections or spam-folder placement tomorrow.
