T-online email client don't receive emails sent through Marketing Cloud

 T-Online email is notorious for repeating delivery problems coming from Salesforce Marketing Cloud domains. Case in point: In the course of an IP warming, we precisely sent 1 (one) email off a new, cold IP to t-online email, and got a blocked bounce message back. This clearly shows, t-online seems to block even if you don't "overpower" / send too much to them. For undisclosed reasons, to my knowledge. So the first thing you have to follow is to make sure you follow the rules carefully. We understand that all other clients have no issue with your emails, but you can never be too sure. If you are sure everything is fine, skip the part between the "=".

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• Make sure that you are not having a messy IP reputation to start with
  
• Cross check that you are not blacklisted or fail at best practices with your emails (tons of factors from DKIM to text/image ratio...). Having an SAP covers the DKIM part, btw., as it authenticates your domain for sending.
  
• If you are sure that you have no issues here (for our intents and purposes, you can ignore missing DMARC, which is often highlighted by mail-tester):
  

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Now, verify what the issue is in your case: Query the _Bounce Data View for the

Just for full picture, query these fields:

• BounceCategory
  
• BounceType
  
• SMTPBounceReason
  
• SMTPMessage
  

We suspect you will find indications of block bounce "due to reputation" or similar reasoning. So our situation likely is: We cleaned in front of our own door, found no issue with reputation, anyway we're blocked. That is the time to raise a support ticket with the deliverability team. Support will likely get in touch with t-online and try to resolve the issue "politically". This is a rather routine thing actually. Normally, you should see the issue resolved in a matter of a day or two.

I hope this helps!