T-Online is a German ISP mailbox (Deutsche Telekom). Across 2,088,148 T-Online addresses verified by BounceZero between April and October 2026, 13.9% were invalid, 0.0% catch-all, 41.2% unknown and 16.4% confirmed valid. Unknown dominates because the gateway defers probes.
The T-Online row is all 2,088,148 addresses verified in the six months to October 2026; the other rows are the benchmarks from the same corpus. Risky and role results fill each row to 100%.
| Segment | Addresses | Invalid | Catch-all | Unknown | Confirmed valid |
|---|---|---|---|---|---|
| T-Online | 2,088,148 | 13.9% | 0.0% | 41.2% | 16.4% |
| All non-freemail domains (business and ISP) | 6,514,884 | 14.5% | 2.3% | 27.0% | 32.6% |
| Business domains only (consumer ISP and freemail excluded) | 1,269,919 | 22.9% | 9.4% | 22.5% | 40.3% |
| All freemail | 1,415,154 | 17.6% | 1.1% | 2.1% | 74.0% |
| All .de addresses | 2,095,440 | 13.8% | 0.0% | 41.2% | 16.5% |
Non-freemail includes consumer ISP mailboxes; a business-only cut is on the benchmarks page. Method and every other cut: Lead List Quality Benchmarks 2026. Risky and role on the T-Online row: 28.5%.
T-Online is the largest German consumer mailbox and a common address on older German B2B and SME lists. Its gateway answers most verification probes with a deferral rather than a yes or no, so the unknown share is the highest of any large provider we see. That is gateway behaviour, not a sign the addresses are dead. The invalid share is close to the business average.
T-Online addresses were issued with Telekom DSL contracts since the late 1990s and many customers keep the mailbox after switching providers, so the domain survives far longer than most ISP mail. On German B2B lists it usually marks sole traders, craft businesses and professionals who never moved to a company domain. German competition law (UWG) treats unsolicited email to anyone, business or consumer, as requiring prior consent, so the segment is a phone-first segment whatever the verification result says.
On confirmed-invalid rate, T-Online ranks 15 of 29 providers in the corpus, just behind Hotmail at 16.5%, ahead of EarthLink at 10.7%.
On unknown rate, T-Online ranks 8 of 29 providers in the corpus, just behind BellSouth at 43.7%, ahead of Yahoo Mail at 39.0%.
On confirmed-valid share, T-Online ranks 18 of 29 providers in the corpus, just behind Yahoo Mail at 16.4%, ahead of BellSouth at 15.5%.
How to read these rates: they are BounceZero's probe outcomes on customer lists over April to October 2026, not a census of T-Online mailboxes. Unknown and catch-all measure what verification could not confirm, which depends on the provider's gateway policy during the window as much as on the addresses. The lists are the ones customers chose to upload, so segments dominated by a few large uploads carry their age and source. Method and limitations: benchmarks page.
Most T-Online results are unknown because the gateway defers verification probes. Nothing in that number says the mailboxes are dead; it says email verification alone cannot clear this segment.
T-Online runs at 13.9% confirmed invalid, ordinary for a list that has not been checked in a while; the failures are closed mailboxes and should be removed.
Catch-all is negligible on T-Online (0.0%); when this provider does return valid, the mailbox accepted the probe itself rather than a blanket accept.
Confirmed valid is 16.4%. Send to that slice from the primary domain and treat the rest of the T-Online segment as unreachable by cold email.
Note the risky and role share: 28.5% of T-Online results landed there, which the four headline columns do not show. Read risky as 'deliverable but with a flag' and test it at low volume before trusting it.
Do not treat unknown T-Online addresses as invalid. Keep them out of cold volume, re-check them after a week, and reach the contact through phone or LinkedIn if the address stays unconfirmed. For German lists, plan on roughly four in ten addresses needing a second channel.
Role addresses such as info@ and sales@ make up 0.0% of T-Online addresses in the corpus. Role addresses are rare here, as expected for a mailbox provider.
All provider and country cuts, with method
Every domain in the benchmark set
Sources ranked by data quality
Thresholds and filtering at this provider
The country segment T-Online belongs to
13.9% of 2,088,148 T-Online addresses verified by BounceZero between April and October 2026 were confirmed invalid, against 14.5% across non-freemail domains and 17.6% across freemail.
41.2%. Unknown means the mailbox could not be confirmed either way, usually because the gateway deferred the probe. BounceZero refunds unknown results; re-check after a week or use a second channel.
Not in practice: 0.0% of T-Online addresses returned catch-all, so a valid verdict means the mailbox exists.
16.4% confirmed valid at verification time. Keep bounces under 2% and complaints under 0.3% on the send, and re-verify the segment before every send.
Upload the file and see valid, invalid, catch-all and unknown per row, with the T-Online segment against this benchmark. 100 free checks a month, unknowns refunded.
Ayoub built BounceZero's 5-stage validation pipeline, its dedicated BGP-announced IP infrastructure, and the Patroni HA PostgreSQL cluster behind every verification. Previously built high-volume email delivery infrastructure. Trained at 1337 Benguerir (École 42 network, 2019). Open-source: bgp_analyzer.
Invalid, catch-all and unknown rates per email provider and country domain, 2026
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