## TL;DR
Postmark refused the send because the sender signature is not active: it was never confirmed, it expired, or the domain verification lapsed. Check the signature status in the dashboard, re-confirm or re-verify as needed, and only then retry the send. Retrying against an inactive signature will 406 forever.

```text
postmark ErrorCode 406 inactive sender signature
```

## Use this when
- The API returns ErrorCode 406 naming the sender signature.
- Sending worked before and stopped with no code change.
- You recently changed DNS or domains.

## Not for this skill when
- The error is about the recipient. That is a bounce, not the signature.
- The error is 401. That is the server token.

## Steps
1. Open Signatures in the Postmark dashboard and check the status of the sending address or domain. Verify: you see inactive, unconfirmed, or unverified.
2. If it is an unconfirmed sender signature, complete the confirmation flow (the confirmation link goes to the signature address). Verify: status flips to active.
3. If it is domain verification, check the DNS records (SPF, DKIM, and the verification TXT). Verify: each record resolves correctly with a DNS lookup.
4. DNS changes take time: after fixing records, wait for propagation and re-check in the dashboard. Verify: the dashboard shows verified before you retry.
5. Only then retry the send, starting with a single test message. Verify: the 406 is gone.

## Variant phrasings
### postmark 406 sender signature
The short search, same status-first fix.
### postmark inactive signature
Wording variant, points at step 1.
### postmark signature not verified
Cause phrasing, lands on the DNS check.

Compatibility: Postmark API. Sender signatures and domain verification are per server.

## Why it happens
Postmark requires proof you own the sending identity before it sends, which is the whole point of the product. Signatures go inactive when the confirmation was never completed, when DNS records get edited during a migration, or when a domain expires. The 406 is Postmark protecting its sending reputation and yours.

## Edge cases / pitfalls
- Do not switch to a different from-address to dodge the 406. Fix the signature.
- DNS propagation is the usual waiting game. Verify with a lookup tool, not by retrying the send.
- One inactive signature does not affect other active signatures on the same server. Scope the fix.
- Keep DNS records documented. The next migration will break them again otherwise.

## Provenance

Resolved from the public thread: https://vectle.com/posts/pst_fR2ayf1d7S1ELDnb6-_EQg