What is the maximum header size for SMTP?

The maximum header size for the Simple Mail Transfer Protocol (SMTP) that is allowed by the standard is 998 bytes. This limit applies to the sum of all header lines, including the SMTP envelope information. Going over this size may cause issues for mail servers and delivery of the email message.

What is the maximum header size for SMTP?

Why Header Size Matters for SMTP

The header of an SMTP email contains important information that tells mail servers how to deliver the message properly. This includes:

  • Envelope information like sender, recipients, routing
  • Subject line
  • Message headers like Date, From, To, CC
  • Custom headers

All of these headers combined must fit within the 998 byte standardized limit for SMTP. If the total size goes over this maximum value, it can cause:

  • Rejection by mail servers due to non-compliance
  • Message delivery failures
  • Email getting flagged as spam

So keeping headers trimmed and optimized is necessary for smooth, successful email delivery.

Checking Your SMTP Header Size

To check the current header size of an SMTP email, you can use the GetHeaders script in PowerShell. Here is an example:

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Get-Content .\message.eml | Select-Object -First 10 | GetHeaders | Measure-Object -Line | Select-Object Lines, @{Name=”Total”;Expression={($_.characters)}}

This will output the number of header lines and total bytes used by the first 10 lines. Other options include using the HEAD method in Telnet or certain SMTP client applications. The goal is to keep the total under 998 bytes.

Key Takeaways:

  • The maximum header size for SMTP is 998 bytes total based on the standard
  • Going over this size can lead to failed delivery or spam filtering of messages
  • Check your current header size with PowerShell’s GetHeaders script or other SMTP tools
  • Optimizing and trimming headers helps meet size limits for good email deliverability

Optimizing Your SMTP Email Headers

Here are some tips for making sure your SMTP headers stay within proper size limits:

Simplify the Subject Line

The subject line takes up a significant chunk of header space, so keep it simple without overstuffing keywords. Clear, concise subjects also improve open rates.

Limit Recipients and Carbon Copies

Each recipient and CC adds to the header size. Reduce lists to essential contacts when possible. Consider using mailing lists for mass messages instead of huge CC lists.

Remove Unnecessary Headers

Don’t include headers like X-SpamThreshold that are not needed and consume space. Most default client headers can be removed.

Shorten Custom Header Keys

If adding custom headers, try to shorten key names as much as possible while still being descriptive.

Compress Value Data

For custom headers with JSON values, compress the data which takes up less space than verbose formatting.

Carefully optimizing and trimming headers helps maximize the amount of space available for important envelope recipient and content data.


The key takeaway is that SMTP has a maximum header size of 998 bytes total for a standard-compliant system. Exceeding this defined limit poses risks of email delivery failures or getting blocked as spam. All mail servers and clients should attempt to keep headers trimmed under this size.

Checking your current header size is easy with freely available tools. And optimizations like simplifying subject line, limiting recipients, and compressing custom headers can help reduce unnecessary overhead. With some diligent attention to header hygiene, you can ensure your SMTP emails get delivered reliably and avoid oversized headaches.

Frequently Asked Questions

  1. What are some main components of an SMTP header?
    Some main components include sender & recipient envelope info, subject line, date, message headers like From/To, and custom headers. All these combine to make up the header.
  2. How is the 998 byte limit calculated exactly?
    The limit sums the entire set of SMTP header lines in bytes, including the trailing CRLF. So 1000 bytes with CRLF would exceed the 998 max.
  3. What is the most common source of big SMTP headers?
    Oversized subject lines and overly long recipient/CC lists contribute heavily to header bloat. Custom headers can also pile on size.
  4. What happens if my header size exceeds 998 bytes?
    Exceeding the limit may cause rejection by recipient mail servers or issues triggering spam filters incorrectly. Delivery cannot be guaranteed.
  5. How can I check the size of headers in Gmail or Outlook?
    In Gmail, enable Show Original to view full header source. In Outlook, File > Properties > Internet Headers displays the verbose header.
  6. What is the maximum number of recipients allowed in an SMTP message?
    The protocol itself does not define a recipient limit, but servers may set restrictions. Limiting recipients helps reduce header size.
  7. Does the header size limit apply to MIME encoded attachments?
    No, the 998 byte header maximum excludes the body and any attachments encoded with MIME multipurpose encoding.
  8. What is the maximum overall SMTP email size with body & attachments?
    SMTP does not inherently limit overall message size including body content and attachments. But mail servers may set max size rules.
  9. What is the average header size for most SMTP mail?
    Average header size for person-to-person SMTP mail tends to run between 200-400 bytes with wider variability for bulk mail and newsletters.
  10. Can I compress an oversized SMTP header to fit?
    Yes, with some optimizations like simplified subject line, fewer recipients, and compressed custom headers, you may reduce size enough without losing vital info.
  11. What should I do if my headers consistently exceed SMTP limits?
    For frequently oversized headers, consider using mailing lists, shorter subjects, more segmented messages, or S/MIME compression features.
  12. Is there any whitespace compression done on SMTP headers?
    SMTP headers follow general email format conventions like CRLF line endings which do not employ whitespace compression techniques seen in HTTP.
  13. What is the maximum number of custom headers I can add to an SMTP message?
    There is no inherent limit on custom header quantity, but adding too many long custom headers can make meeting size limits difficult.
  14. Is it OK to just increase the SMTP server’s maximum header size limit?
    Most administrators do not recommend widening this limit as going beyond the standard 998 bytes can lead to issues with Email serving ecosystem.
  15. How long can the subject line be without risking header size problems?
    Some recommendations advise keeping subject lines around 50 characters or less. Subject length rapidly increases SMTP header sizes.
  16. Should I worry about header size limits for internal corporate email?
    Header limits can affect internal mail as well as external delivery. But corporately controlled mail systems have more flexibility to relax restrictions.
  17. Where is the 998 byte header limit officially defined for SMTP standard?
    RFC 5321 section defines compliant SMTP servers must accept 998 bytes of header including trailing CRLF per line.
  18. How Does SMTP Header Size Compare to HTTP Headers?
    HTTP header size limits are typically much higher than SMTP email headers. The HTTP/1.1 spec does not formally define a specific limit, but common web servers support headers ranging from 4-16KB or more. Some optimizations similar to SMTP apply to keeping HTTP headers small, such as compressing cookies, using short descriptive header names, and removing unnecessary headers.
    But overall HTTP has less intrinsic size constraints for headers than SMTP mail. This allows heavier use of expansive metadata, cookies, and custom headers in web requests. However large unwieldy HTTP headers can still pose issues for browsers or other client applications and degrade performance. Finding the right balance for needed functionality vs trim efficiency is key for both protocols.

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