Most companies aggressively filter inbound mail: spam, phishing, malware. What they send out, they rarely look at more closely. Yet that is exactly where the reportable GDPR incidents of the last few years have originated – an inadvertently attached HR file, a misdirected reply-all with a customer list, a developer mailing an API key to a vendor. Outbound DLP starts exactly there.
What DLP Actually Does
A modern outbound DLP system inspects every outbound email from an authenticated user before handing it to Postfix, against a configurable policy set:
- Structured detectors: Credit card numbers (with Luhn validation), IBANs (mod-97), tax IDs, US SSNs – with format validation rather than blind regex that flags every 13-digit number as a card number.
- Secrets detectors: AWS access keys, GCP service account JSON, GitHub PATs, OpenAI keys, generic high-entropy strings – the most common sources of accidental credential leaks.
- Classification markers: Keywords like "confidential", "strictly confidential", "internal distribution" – configurable per domain and business unit.
- Attachment inspection: Office documents, PDF text, and ZIP contents are fed into the same pipeline – not just the mail body.
The Big Problem: False Positives
Naive DLP solutions block so many legitimate emails that IT disables the quarantine after three weeks. SecTepe.Comm takes two routes to keep the false positive rate low:
- Per-policy action instead of global block: Each rule can trigger "log only", "warn user", "quarantine", or "block". An IBAN from an accounting domain is logged, a PEM private key line from the marketing mailbox is hard-blocked.
- Redacted snippets instead of full hits: The match view for admins only shows the masked hit (e.g.
4111-XXXX-XXXX-1111) – compliance-friendly investigation without a new data leak vector.
Multi-Domain & Per-Domain Policies
Outbound DLP only delivers full value when it can be configured differently per domain. A holding company with five subsidiaries wants different classification rules for the tax consultancy than for the sales subsidiary. The SecTepe.Comm domain registry allows exactly this granularity – down to per-sender and per-recipient exceptions.
Four-Eyes Release: The Missing Safety Net
Even the best DLP rules sometimes quarantine a lawyer's PDF that has to go out. The alternative to "the admin decides alone" is a four-eyes approval flow. Releases for high-risk verdicts (DLP hit, sandbox threat, policy violation) are only possible if a second operator actively approves them in the UI. This is also the ISO 27001 evidence for "segregation of duties" – without Excel lists or Slack pings.
What DLP Does Not Replace
A DLP system is not a replacement for awareness, clear classification policies, or sensible secrets handling (e.g. a vault instead of PEM files in emails). It is the last technical defense line catching human errors – and insights from the matches often provide valuable feedback for awareness training and process adjustments.
Conclusion
Outbound DLP belongs in every serious mail security concept. What matters is not that "DLP is on", but how precise the rules are, how clean the action escalation runs (warn → quarantine → block), and how frictionless the release process is. SecTepe.Comm delivers the pattern set, the UI, and the four-eyes safety net – as part of the same platform that already filters inbound mail.