What is a header field limit?
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A header field limit is the maximum length allowed for email headers, both per line and for the entire header section. Cross this limit and mail servers will truncate your email or reject it outright.
RFC 5322 (the standard that defines email format) recommends keeping individual header lines under 78 characters for readability. The hard limit is 998 characters per line. Beyond that, the line must be folded (broken across multiple lines) or it'll break.
For the total header section, there's no official RFC limit, but most mail servers enforce practical caps between 32KB and 256KB. If your headers balloon past that, it's usually a sign something's wrong (a misconfigured tracking system adding redundant headers, or a spam attempt with thousands of routing hops). Mail servers tend to reject these emails on sight.
The most common cause of oversized headers: ESPs that inject dozens of tracking and routing headers into every message. If you're sending through SendGrid, Mailchimp, or any platform that adds its own headers, you're usually fine. But if you're building your own SMTP setup or routing through multiple relays, check your raw headers occasionally. Our Email Header Analyzer will flag oversized sections and show you which fields are eating up space.
If you hit this limit in the wild, the symptoms are rejection bounces ("message too large") or silent truncation where the receiving server just cuts off your headers mid-flight. Fix it by trimming custom headers, reducing relay hops, or debugging whatever's injecting redundant data.
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