| T-Online | |
|---|---|
| Domains | t-online.de, magenta.de |
| Mail routing | mx00.t-online.de and its siblings, operated by Deutsche Telekom. |
| What we can resolve | We resolve some mailboxes here |
How T-Online answers a probe
What the server does when we open a connection and ask about one address.
- Unknown recipients return a 550, and the response is dependable.
- Telekom enforces connection limits firmly and expects correct reverse DNS on the connecting host before it will talk properly at all.
- A probe from an IP without a matching PTR record gets a worse reception here than almost anywhere else.
- The magenta.de domain is newer branding on the same service.
The part that catches people out
What we do about it
Our probe hosts carry matching forward and reverse DNS, which matters more at Telekom than anywhere else in Europe. Where a connection is still refused we report unknown rather than treating a rejected connection as a rejected address.
Sending to T-Online mailboxes
Verification is one step. These are the things that decide whether the message lands.
- Telekom expects proper authentication and correct reverse DNS from senders too, and enforces it at the connection.
- The subscriber base is older and broader than Web.de's, and includes a lot of household addresses.
- German ISP addresses decay with the broadband contract, like their French and American equivalents.
The tosa@ message, word for word
Telekom's refusal is one of the most recognizable lines in email. A server it doesn't know gets a 554 at connection time with text along these lines: "IP=203.0.113.25 - A problem occurred. Ask your postmaster for help or to contact tosa@rx.t-online.de to clarify." No recipient has been named yet. The message is about the machine that called.
Say you launch a newsletter from a new dedicated IP and every one of your 1,800 t-online.de subscribers fails with that line on the first day. Your list is fine, and the block sits in front of it. Telekom wants the operator of the sending server to write to that address and identify themselves, and it has long expected the sending domain to show a proper legal notice with contact details. Once the IP is cleared, the same 1,800 addresses accept mail. Suppressing them on day one would have thrown away the whole segment over paperwork.
How to tell a door refusal from a mailbox refusal
Look at where in the conversation the error landed. A 554 before EHLO, carrying your IP address in the text, is the door. The 550 that comes straight after RCPT TO is the mailbox. Any verifier or sending log that records the stage as well as the code lets you separate them in seconds, and any tool that records only "failed" doesn't.
Two more details will help you plan a German send. magenta.de publishes its own MX names, mx00 through mx03.magenta.de, while t-online.de uses mx00 through mx03.t-online.de (both checked September 19, 2026), so group them deliberately instead of assuming your tool does. Telekom also offers free t-online.de mailboxes to people with no Telekom contract, so don't assume every address there ends when a broadband deal does. Ask a verification vendor one direct question: what do you return when Telekom refuses the connection? You're listening for the word unknown.
Questions people ask
Why does T-Online reject my connection?
Usually because the connecting IP has no matching reverse DNS record. Telekom checks that before it will engage properly, for senders and for verifiers alike.
Can t-online.de addresses be verified?
Yes, once the connection is accepted. They reject unknown recipients properly. The infrastructure requirements sit in front of the answer rather than replacing it.
What is magenta.de?
Deutsche Telekom's newer brand name on the same mail service. The addresses route identically.