Verify gmx.de addresses

How Kavik verifies GMX addresses: United Internet's user-unknown replies (UI r1601 class) map to invalid, not to IP blocks.

GMX (gmx.de, gmx.net, gmx.at, gmx.ch) belongs to 1&1 / United Internet, whose mail servers answer RCPT probes with terse numeric replies. Reading those replies wrong is the classic way to get GMX verification wrong.

How Kavik verifies GMX

  • United Internet’s user-unknown reply class (UI r1601) maps to invalid. It is a per-mailbox answer, not an IP block, and Kavik classifies it as such.
  • True connection-level blocks (which also happen on this platform) are classified separately and become unknown, not invalid, so a bad egress day never deletes real mailboxes from your list.
  • Catch-all siblings on the same platform are measured and labelled.

What the verdicts mean here

  • Valid: the mailbox was accepted under United Internet’s real reply classes.
  • Risky: catch-all or ambiguous platform behavior.
  • Invalid: a user-unknown reply for this specific mailbox.
  • Unknown: connection-level refusal this time; never billed.
curl "https://api.kavik.email/api/v1/verify?email=jane@gmx.de" \
  -H "X-API-Key: eva_your_key"

See single verification for the full response shape.