T-Online Bounce Messages: 421, 550-5.7.0 and 554 Explained, Plus the 3% Bounce Limit

T-Online's postmaster page sets a clear bar: keep bounces at 3% or less at peak and 1.5% or less on average, and remove any address that hard bounces. It also documents the three rejections you are most likely to meet: 421 for too many connections, 550-5.7.0 for spam or a virus, and 554 for a blocked IP. This post goes through them in our words, from the page at postmaster.t-online.de as read on 9 November 2026. The first one to fix is usually the list.

Check a t-online.de address →

The messages at a glance

Message	What T-Online's page says	Fix is on
421 Maximum parallel connections for your IP-Address reached	Too many simultaneous connections	Your server
4xx "System temporär nicht verfügbar"	Temporary: unusually high load, or an exceeded IP session limit	Retry later
550-5.7.0 Message considered as spam or virus, rejected	Content looked like spam or a virus	Content, list
554 IP=[SENDER-IP] - A problem occurred	IP blocked after observed unusual mail volume	Your sending
Unknown user (hard bounce)	The address does not exist	Your list

The bounce-rate rule

This is the number to know. The page says bounce rates should be at most 3% at peak and no more than 1.5% on average. It says higher rates suggest outdated addresses or missing double opt-in, and the page treats that as abuse.

Take a send of 20,000 messages to t-online.de addresses. At 1.5% average, that is 300 bounces over time. A list full of addresses from old sign-ups can pass 3% on its own. Use the bounce rate calculator on your last send to see where you stand, and the email list bounce rate estimator before the next one.

421: too many parallel connections

The documented text is "421 Maximum parallel connections for your IP-Address reached". You opened more simultaneous connections than T-Online accepts from one IP. It gives no number. The page's advice is to reuse connections and to tune "peer concurrency" and "destination concurrency" in your mail server. It also asks senders to keep simultaneous TCP/IP connections to a minimum.

A 421 is temporary, so mail should be retried. If you use an email platform, lower the sending speed to T-Online domains and let the platform retry. The email send rate calculator helps plan a send.

The page also describes generic 4xx messages such as "System temporär nicht verfügbar", linked to high server load or an exceeded IP session limit. Same treatment: retry, and spread the send.

550-5.7.0: considered spam or virus

The page shows "550-5.7.0 Message considered as spam or virus, rejected" and says to stop sending until the problem is fixed. Don't resend in a loop.

What to do:

1. Check your content and links. Look for anything that could look like a virus or phishing: shortened links, odd attachments, mismatched sender and link domains.
2. Check your authentication with the email domain health checker.
3. Check the list. Mail to old, never-confirmed addresses is part of what makes sending look like abuse.
4. Send a small test to your own t-online.de account before a full resend.

554: IP blocked

The example on the page is "554 IP:87.167.75.190 - A problem occurred." The explanation is that the IP is blocked because of previously observed unusual mail volume. The page lists [email protected] as the contact, and [email protected] if your system is suspended from delivering to t-online.de addresses.

What to do:

1. Look at your volume. A sudden jump to t-online.de, or a long list sent in one burst, is the pattern the page describes.
2. Check blocklists with the IP blacklist check; a 554 can arrive with or without other listings.
3. Contact postmaster with the full bounce and what changed. Don't resume the same send first.
4. Come back slower, with a cleaned list.

What T-Online asks senders to do

From the requirements and list-hygiene sections of the page, in our words:

• Use a valid envelope sender so bounces can reach you.
• Follow the relevant RFCs and forward-confirmed reverse DNS. Check yours with the reverse DNS lookup.
• Send bulk mail only with explicit, personal consent, typically confirmed by double opt-in.
• Offer a direct unsubscribe in every automated mail or newsletter.
• Delete or block addresses without verifiable double opt-in.
• Delete or block addresses with no contact for more than three months.
• Delete or block addresses that return a hard bounce.
• Use dedicated IPs when sending on behalf of others, especially for clients whose reputation is in doubt.
• A client that doesn't send HELO within 6 seconds is disconnected, and messages are limited to 50 megabytes in total.

The three-month rule is stricter than most. If your list has T-Online subscribers who haven't engaged in longer, a sunset policy gives you a clean way to follow it.

Unknown users and backscatter

The page says inbound mail to non-existent recipients should be rejected during the SMTP conversation, so the receiving side never has to bounce it later. That is what keeps backscatter out. For you, the lesson is that an unknown-user rejection is a final answer: the address must come off your lists.

Hard bounce vs soft bounce explains the difference, and 550 5.1.1 user unknown covers the same problem at providers that use enhanced codes.

Keep your T-Online bounce rate under the line

1. Pull the t-online.de rows out of the file and check them first.
2. Verify the whole list at /verify. Invalid rows go to suppression for good. Risky rows go into a small, slower group, sent after the valid ones.
3. Check single addresses with the T-Online email checker if you only have a few.
4. Re-verify older lists before reuse. How often to verify an email list gives a schedule.
5. Watch the rate per send, not just in total. A single old segment can push a send past 3%.

For other European consumer providers, see GMX and WEB.DE bounce messages and Orange bounce codes.

Quick answers

• Is 421 a bounce? No. It is temporary and should be retried, slower.
• Can I stay under 1.5% by cleaning only once? Lists age. The rate climbs again unless you re-check before big sends.
• Does T-Online use greylisting? The page says it is used only against repeatedly conspicuous systems and suspected abuse.

Try it on your own list

Upload a sample at /verify. The first 100 checks are free with a Google sign-in, no card.

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*Fair use: the first 100,000 checks a month at full speed, then slower, never cut off.