---
summary: Format and meaning of the X-ScaiLine header that every processed message
  carries.
order: 2
title: X-ScaiLine header
path: reference/x-scailine-header
status: published
---

# X-ScaiLine header

Every message that passes through ScaiLine (milter, hosted relay, ScaiSend stage) and belongs to
an organisation ScaiLine knows leaves with exactly one `X-ScaiLine` header, unless the message was
passed through unmodified. The header tells you, per delivered message, what ScaiLine did. Mail
from a domain that no organisation has registered passes the milter without a header.

## Format

```
X-ScaiLine: v=1; t=<tenant-id>; b=<bundle-version>; o=<outcome>; ts=<unix>; mac=<base64>
```

| Field | Meaning |
|---|---|
| `v` | Header format version. Currently `1`. |
| `t` | Organisation id (`tnt_…`). |
| `b` | Version of the compiled bundle that was used; `0` when no bundle applied (skip). |
| `o` | Outcome: `stamped`, `client`, `skip` or `bypass` (see below). |
| `ts` | Unix time of processing. |
| `mac` | HMAC-SHA256 truncated to 128 bits over `t|b|ts|Message-ID`, keyed with a per-tenant secret held in ScaiVault, base64. |

## Outcomes

| `o=` | What happened to the body |
|---|---|
| `stamped` | The signature variant (`external.new`, `external.reply`, `internal.new`, `internal.reply`) was inserted into the HTML and text parts. |
| `client` | The Outlook add-in or a native integration had already inserted the signature (it set `X-ScaiLine-Client`); only the remaining components (legal footer, banner) were applied. |
| `skip` | Deliberately left alone: signed or encrypted mail, calendar traffic, automatic replies and reports, a sender without a profile, a skip rule, already processed, or no signature for this recipient scope. Body unchanged, header present. |
| `bypass` | The engine could not safely touch the message (oversize, deadline exceeded, unknown MIME structure, internal error). Body unchanged. A bypass **does not add the header**: the message is byte-identical to the input. |

In **shadow mode** (hosted relay only) the outcome is computed and recorded, and the original
message is delivered.

## Why the MAC

The header doubles as loop prevention: a message that already carries a *valid* `X-ScaiLine`
for the same tenant is treated as already processed and skipped. Only a valid MAC is trusted.
The Exchange Online transport-rule exception can only look at the header's text (it matches
`v=\d`), so a sender who writes a fake header can at most suppress their own signature.

## Reading it

Message headers are visible in every client ("View source", "Show original", or `less` on the
`.eml`). For support questions, quote the whole header line; the tenant id, bundle version and
timestamp identify the exact template and directory state that was in force.

See [Signature missing](troubleshooting/signature-missing) for what to do per outcome.