Changelog
Changes that affect developers: the REST API, the SMTP relay, webhooks, domain setup and these docs. Newest first. For new features in the Koltrix app, see the product changelog.
/api/v2 is the stable API. Changes to it are additive: new fields and
endpoints may appear, but existing fields, status codes and behaviour don't
change without a new version and notice.
2026-10-03
AI assistants (MCP). Connect Claude, ChatGPT, Cursor and other MCP apps to
your Koltrix inbox at https://mcp.koltrix.com/mcp. Assistants can search,
read, organise and draft; they can never send, forward or delete. Sign-in is
OAuth, with per-connection permissions, revocation and an admin switch. See
AI assistants.
Docs. Every page was checked against the API's code and corrected. New pages: Errors, Deliverability and suppressions and this changelog. Known gaps are now marked "Current limitation" where they apply.
2026-10-02
Five DNS records for new domains. A new domain now asks for an ownership
record (_koltrix), MX, SPF (include:_spf.koltrix.com), its own DKIM key
(kx1._domainkey) and DMARC. If your domain already has an SPF record, Koltrix
shows a merged one. Domains verified before this keep working unchanged and
show the new records as a recommended upgrade; once their kx1._domainkey
record checks out, their mail is signed with their own key automatically. See
Custom domains and DNS.
Domain claims. A domain can be pending in more than one workspace, so nobody can block its real owner by adding it first. The first workspace whose own ownership record appears in DNS verifies it.
2026-10-01
Developers hub. API keys, SMTP settings, webhooks and delivery logs are gathered under Developers in the app, with a copy-and-run quickstart.
2026-09-29
Send quotas on the API and SMTP relay. Sends through POST /api/v2/emails
and the SMTP relay count against your plan's API send allowance, per recipient.
Over the allowance the API answers 429 with "code": "quota_exceeded" and
resets_at, and the relay answers a temporary 452, so mail servers retry
rather than drop the message. See Limits and quotas.
2026-09-01
Per-key rate limit. /api/v2 allows 60 requests per minute per API key and
answers 429 with Retry-After above that.
2026-08-31
SMTP relay limits. At most 1,000 recipients per message (452 4.5.3 above
that, so the rest can be sent in another message), and a message over 25 MB is
refused with a permanent 552 5.3.4 instead of a retryable error.
2026-08-30
Idempotency without duplicates. The Idempotency-Key is claimed before a
message is queued, so a retry that races the original can no longer send a
second copy. A retry during the original answers 409; if the idempotency
store is unavailable the API answers 503 and sends nothing.
Bounces that come back later. A permanent bounce report that arrives after
the message was sent now marks the message bounced and adds the address to
the suppression list.
Unsubscribes suppress. Unsubscribing, from a link or through the API, adds the address to the workspace's suppression list, so no list, sequence or transactional send mails it again.
2026-07-30
Display names in From. "from": "Acme <[email protected]>" is accepted by
POST /api/v2/emails.