How SMTP Relay works

SMTP Relay sends the mail your software writes: receipts, password resets, alerts and newsletters. Your application hands each message to the relay, and the relay signs it for your domain, delivers it and records what happened. SMTP Relay only sends. To receive mail on a domain, use Mail Hosting.

Over SMTP

Point any SMTP library or application at the relay's SMTP host on port 587 (STARTTLS) or 465 (TLS), and sign in with an SMTP credential: a username and password the relay creates for one application. See Send over SMTP with credentials.

Over HTTPS

Post a JSON message to the send API with an API key in the X-Api-Key header. See Send email over HTTPS.

Both need a verified sending domain on the relay, and the From address must be on it. The HTTPS API sends through one of the relay's SMTP credentials, so create at least one credential even if you only use the API.

1. The relay checks the sender, the monthly allowance and the hourly limit, and drops any recipient on the suppression list.
2. It signs the message with your domain's DKIM key and delivers it.
3. Each step becomes an event: accepted, delivered, deferred, bounced or complaint. You can look events up for up to 90 days, or receive them with webhooks.
4. A bounce or a spam complaint adds the address to the suppression list, so the relay stops sending to it.

Each message is either transactional or marketing. Marketing mail gets one-click unsubscribe headers. See Transactional and marketing mail.

• Emails a month: the plan's allowance. Paid plans accept a buffer beyond it; the free plan stops at its allowance. The count resets on the 1st of each month.
• Sending domains: a number, or unlimited. An unlimited plan still stops at 1,000 domains.
• SMTP credentials: a number, or unlimited. An unlimited plan still stops at 200 credentials.
• Dedicated IPs: some plans include a dedicated sending address, and a relay whose plan sends 100,000 emails a month or more can add one.

The hourly limit is separate from the plan. Every new relay starts with a small one, and it rises as the relay sends clean mail. How SMTP Relay sending limits work covers the allowance, the buffer and the hourly limit.

In the dashboard, go to Email, open the relay, then the Overview tab.

The three figures at the top are Emails this month against the allowance, and the Sending domains and SMTP credentials used against the plan. Once the relay has sent 80% of its plan's emails for the month, an alert above them says how much is left; see Sending limits.

Deliverability

The bounce and complaint rates over the recent days the card names, the relay's health checks, and the Default message category. See Sending reputation and deliverability.

SMTP endpoint

The SMTP host name to connect to, with its ports.

Recent activity

The latest events from the last seven days. All events opens the Events tab.

Service

The relay's Status, Plan, Region, the date it Renews, when it was Created and its Service ID.

Tags

Labels you add to find the relay. See Tags.

Hourly limit

The Current limit, the Next step it rises to, and when and how it was Set. Request limit increase… asks for more; see Request a higher hourly limit.

Sending IPs

Whether the relay sends from addresses we share between customers or from dedicated addresses. Each dedicated address shows its reverse DNS name, its hourly cap, and a Warming up badge while it warms up. Order dedicated IP opens the order page for another dedicated address where the plan allows one (Order a dedicated sending IP).

Domains

Each sending domain and its verification status.

Read the relay's summary:

curl https://api.coritan.com/api/v1/client/smtp-relay/4812 \
  -H "Authorization: Bearer $CORITAN_TOKEN"

The fields you need most:

kind

relay for SMTP Relay.

status

active once the relay is ready.

month_to_date_sent

Emails counted this month.

limits

The allowance, the buffer, what is left, and the hourly limit with its next step. Sending limits lists every field.

counts

relay_credential and domains, the credentials and sending domains the relay holds.

default_category

transactional or marketing, the category of a message that does not set one.

dedicated_ips

Each dedicated address, with address, ptr, warming_up (true until its warm-up ends), hourly_limit, enabled and addon_service_id (set when you bought it as an add-on).

cluster

The region the relay runs in: region, its code, and region_name, such as Frankfurt, or null when we have no name for it.

hostnames.smtp_host

The host to connect to over SMTP.

sending_frozen, review_flag, reputation_note

Whether sending is paused or under review, and why.