What is a header field limit?

A header field limit defines the maximum allowed length for a header line or overall header section. Different mail servers enforce different limits, but excessively long headers can cause truncation or rejection.

While not a strict rule, RFC 5322 recommends that individual header lines be kept under 78 characters for readability and must not exceed 998 characters. There is no official RFC limit for the total header section, but most modern mail servers enforce practical limits between 32KB and 256KB. An email exceeding those sizes usually indicates a misconfigured system or spam attempt and will likely be rejected.

If your ship’s manifest grows too long, the harbor master might simply refuse it.