Security & Methodology
Reviewed: 18 September 2026
Your email list is one of the most sensitive things you own. This page describes exactly how ListGuard handles it — the data flow, what is and is not stored, and the limits of every check. Where an exception exists, we name it.
The data flow
1. Upload. Your CSV travels over an encrypted connection to our application on European infrastructure (Contabo, Germany).
2. Core cleaning. Syntax, duplicates, dead domains, disposable and role addresses, blacklist and suppression matching run in one pass on our own machines.
3. Optional external verification. Only if you explicitly connect and run ZeroBounce or Hunter.io, the addresses you choose to verify are sent to that provider. Without your key, no addresses ever leave our infrastructure.
4. Download and deletion. The moment your clean list and report are ready, your uploaded file is removed from our systems.
What is stored — and what is not
Not stored: the addresses in your uploaded list. Core cleaning keeps counts (how many were removed, and why), not your contacts.
Stored: your account data, the counts of your runs, and — only if you explicitly save them — your suppression lists, so future runs keep protecting those addresses.
Infrastructure and subprocessors
The application and database run on Contabo in Germany. Encrypted backups of the database are stored on AWS in Frankfurt; the database does not contain your uploaded list. Membership checks for customers arriving through a partner channel are performed by aMember (by email match, on demand). ZeroBounce or Hunter.io are involved only when you connect your own key. No third-party analytics, advertising or tracking runs on this site.
Encryption
Every upload and download travels over TLS. Passwords are stored as bcrypt hashes. Backups are encrypted at rest.
Methodology and limitations
Checks are explicit about what they measure: syntax validity, domain/MX availability, disposable-domain membership, catch-all behavior and duplicate/suppression matching. Mailbox-level existence is only assessed through the optional external verification layer. No check is a guarantee against future bounces, and catch-all answers are inherently uncertain. We state these limits on every result rather than oversell certainty — see the verification layers for the full breakdown.
Found an issue?
Report security or privacy concerns to us through the contact page and we will respond promptly.