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Probe type · Pro+

Email round-trip probe

Proves your mail actually arrives — submission, transport, retrieval, and authentication — instead of just confirming the SMTP port is open.

Why you need it

An Email round-trip probe is the only check that proves your mail plumbing actually delivers, end-to-end. A TCP probe on port 587 only tells you the relay is listening; a working SMTP host that quarantines its own outbound messages because DKIM drifted looks identical to a healthy one. This probe sends a real test message, waits for it to land in a real mailbox, and (optionally) checks that the receiver's authentication headers come back green.

Three depth modes layered into the same probe:

  • Mode A — SMTP submission only. Connect, EHLO, STARTTLS, AUTH, MAIL FROM / RCPT TO / DATA / 250. Proves the relay is up and accepts the test recipient. Sub-second typical.
  • Mode B — Full round-trip. Mode A plus IMAP or POP3 retrieval. Polls every 30 s (configurable) until the message lands or a 9-minute budget exhausts. Catches misrouted MX, broken alias rules, queue backlog, anti-spam quarantine.
  • Mode C — Round-trip plus authentication. Mode B plus parsing of the receiver's Authentication-Results header — SPF, DKIM, DMARC verdicts surfaced individually. Each verdict has its own per-probe severity. Falls back to in-process DKIM verification (MimeKit) when the receiver doesn't add the header.

Every check records five separate latency measurements: SMTP connect+TLS, SMTP auth, SMTP submission, round-trip transit, and mail retrieval. A hot spot in any one leg bumps just that step into Degraded; the rest stay green. All SMTP and retrieval credentials are encrypted at rest with AES-GCM.

Where it pays off

The check for any team whose product or business depends on email delivery:

  • SaaS transactional mail — submit through SendGrid / Postmark / SES / Mailgun, retrieve from a Gmail or Workspace inbox. Mode C catches DKIM-rotation drift the moment Gmail starts marking your mail as spam.
  • Self-hosted Postfix / Exchange — Mode B is the canonical "is mail actually flowing?" check.
  • Disaster-recovery validation — point Mode B at the failover MTA weekly so you find out it's broken before you need it.
  • Compliance / audit evidence — Mode C's per-verdict history doubles as proof that SPF / DKIM / DMARC alignment stayed green across the audit window.

Not the right choice when: you only need to know the SMTP port is open — use the TCP probe with TLS for that.

Configuration parameters

Field Type Required Default Description
Name string Required — Human-readable label for the probe.
Mode SmtpOnly | RoundTrip | RoundTripPlusAuth Required SmtpOnly Depth level. Field surface scales — retrieval fields appear from Mode B; auth severities from Mode C.
SMTP host string Required — Submission host. Internal hostnames rejected at save time (SSRF guard).
SMTP port integer Optional 587 Default 587 for STARTTLS; 465 for implicit TLS.
SMTP security StartTls | ImplicitTls | None Optional StartTls TLS posture for submission.
SMTP username string Required — AUTH username for the submission relay.
SMTP password string (encrypted at rest) Required — AUTH password. AES-GCM encrypted and never logged.
From address string (email) Required — Used as MAIL FROM and in the RFC 5322 From header.
To address string (email) Required — RCPT TO destination. In Modes B/C this is the mailbox the probe polls for retrieval.
Retrieval protocol Imap | Pop3 Required if Mode != SmtpOnly Imap Protocol used to retrieve the test message. IMAP recommended.
Retrieval host string Required if Mode != SmtpOnly — IMAP / POP3 host.
Retrieval port integer Optional 993 Default 993 for IMAP implicit TLS, 995 for POP3 implicit TLS.
Retrieval username / password string + encrypted string Required if Mode != SmtpOnly — Credentials for the retrieval mailbox. Password is AES-GCM encrypted.
Retrieval folder string Optional INBOX IMAP folder to SELECT. Ignored for POP3.
Max round-trip wait (seconds) integer (60-540) Optional 540 Hard ceiling on how long the probe polls before declaring the message lost.
Round-trip poll interval (seconds) integer Optional 30 How often to re-check the mailbox during the wait window.
SPF / DKIM / DMARC failure severity Ignore | Degraded | Down (per verdict) Optional (Mode C only) DKIM/DMARC: Down; SPF: Degraded Mode C — independent severities per verdict. DKIM/DMARC default Down (deliverability breaks); SPF defaults Degraded (softfails common in multi-relay).

Available on Pro+. Already on StatusPulse? See the full config in Help →

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