What are common formatting pitfalls in email headers?
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Email headers are where things like "From:", "To:", and "Subject:" live. They're formatted in a very specific way (defined by RFC 5322, the internet standard for email structure), and even tiny mistakes can cause receiving servers to reject your messages or flag them as spam.
The most common mistakes:
- Missing the colon. Every header field needs one. "From sender@example.com" won't work. It needs to be "From: sender@example.com".
- Wrong line breaks. Headers must end with a carriage return AND a line feed (written as \r\n in code). If your system only uses line feeds (\n), some mail servers will reject the entire message. This happens more often than you'd think when developers hand-code SMTP.
- Extra spaces. A space before the colon, or random spaces inside field names, breaks parsing. "From : sender@example.com" is invalid. So is "Fr om: sender@example.com".
- Unquoted or badly quoted values. If a display name has special characters (commas, quotes, parentheses), it needs to be properly quoted. "From: John Doe, Jr. <john@example.com>" will confuse parsers. It should be "From: \"John Doe, Jr.\" <john@example.com>".
- Duplicate fields that shouldn't be duplicated. You can have multiple "Received:" headers (mail servers add them as the message travels), but you can't have two "From:" headers or two "Subject:" headers. Servers will reject that.
- Headers that are too long. Each header line should be 78 characters or less. If you need more, you fold the line by starting the next line with a space or tab. Not doing this can cause truncation or rejection.
If you're using an ESP like Mailchimp, SendGrid, or Postmark, they handle header formatting for you. You won't run into these issues unless you're adding custom headers or manually constructing messages via SMTP.
The main symptom of header formatting errors is hard bounces with vague error codes, or mail that silently disappears. If you're seeing "message rejected due to policy violation" or "syntax error in header" bounce messages, check your headers. Our free email header analyzer will show you exactly what receiving servers see, including formatting issues.
One last thing: if you're building your own mail sending infrastructure or debugging bounces, learn to read raw headers. That's where these errors show up.
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