Mailbox provider · T-Online
How to verify a t-online.de email address
T-Online addresses usually come back inconclusive from a mailbox check. On 12 October 2026 the t-online.de mail server mx01.t-online.de refused our checking server at the greeting stage, citing the connecting server's reputation. A refusal like that says nothing about whether the address exists.
Mailbox checks: Hard to verify. The server refuses connections it does not trust before answering about the mailbox.
T-Online at a glance
| MX hosts | mx00.t-online.de, mx01.t-online.de, mx02.t-online.de, mx03.t-online.deObserved 12 October 2026 (dig MX) |
|---|---|
| Answer to a made-up address | Connection refused at the greeting: 554 None/bad reputation. Ask your postmaster for helpObserved 12 October 2026 (SMTP probe of a made-up recipient, no message sent) |
What T-Online's server says about a missing mailbox
T-Online's server replied 554 None/bad reputation. Ask your postmaster for help. That refusal happened at the greeting stage, before the checking server could name any recipient, so it is a verdict on the connecting server, not on the address. SimpleVerifier returns these as risky with "Mailbox check unavailable" and never as invalid, because treating a refusal as "mailbox not found" would delete real customers from your list.
Servers screen connections this way to keep spam out: missing reverse DNS, a low sending reputation or a blocklist listing are the usual triggers. The same screen applies to every checking service that connects from a shared or new IP, which is why t-online.de results differ between verifiers. SMTP verification false positives explains how much to trust each answer.
The reply asks the connecting server's postmaster to contact T-Online to have the reputation reset, so the block is about the checking server rather than any address.
Cleaning t-online.de addresses you can't check
Treat inconclusive t-online.de results as unknown, not bad. The checks that don't need the mailbox still work: format, typo domains, duplicates and whether the domain can receive mail. The free list health check runs those in the browser.
For the rest, use the address's history. An address that signed up, bought or replied recently is very likely live. For old or bought data at t-online.de, send a small batch first and suppress anything that hard-bounces. If the t-online.de share of your list is small, it's often simpler to keep those rows out of a high-risk campaign than to guess.
Live DNS check for t-online.de
These are t-online.de's own sending records, refreshed daily. They describe mail sent from the domain, not whether a given address at it exists.
3 of 5 checks passed
60%Blocklist status
Not listed on SURBL, SpamCop
DKIM — message signing
No DKIM found on common selectors — it may use a custom one
DMARC — spoofing policy
DMARC present but p=none, so nothing is enforced
Mail servers (MX)
4 mail server(s), primary mx01.t-online.de
SPF — authorised senders
SPF present, soft fail (~all)
Common questions
- Can I check if a t-online.de address exists without emailing it?
- Usually not from a checking service. On 12 October 2026 T-Online's server refused our checking connection at the greeting stage, before any mailbox question, so the result is inconclusive rather than valid or invalid.
- What mail servers does t-online.de use?
- On 12 October 2026, t-online.de published these MX hosts: mx00.t-online.de, mx01.t-online.de, mx02.t-online.de, mx03.t-online.de. You can look them up yourself with the free MX lookup tool.
- Should I delete t-online.de addresses that come back risky?
- No. For t-online.de, risky reflects how the server treats checks, not the address. Remove the ones that fail format, typo-domain and duplicate checks, then send to the rest in a small batch and suppress whatever hard-bounces.
Check a T-Online address
The free checker runs the mailbox check on one address and shows the server's reason, so you can see which of the cases above applies.