Hey @Frank-DOGO 

I’m sorry to hear that you’re experiencing this issue surrounding @t-online emails bouncing and thank you for sharing it with the Community!

I would first recommend taking a look into the Meeting the T-online.de Inbox Placement Requirements article provided by @cassy.lee within this thread. Our team was able to gather more information from T-Online which has outlined their inbox placement requirement below:

• You must use a dedicated sending IP and have enough consistent volume to keep the IP warm (100,000 emails per month). It sounds like perhaps instead of a dedicated sending IP, you were able to get a dedicated sending domain. For clarity, a Dedicated Sending Domain uses the root domain of a business, instead of a root domain supplied by Klaviyo. A Dedicated Sending IP address gives you full control of your email sending, so you're fully responsible for the reputation of your own IP.
• Enable double opt-in for your lists
• Your Sender Policy Framework (SPF), DomainKeys Identified Mail (DKIM), Reverse DNS (rDNS; i.e., your domain for dedicated IP), and your sender email domains in Klaviyo (the from-address for each email send) must all align

Additionally, I would recommend reaching out directly to T-Online’s Support Team at postmaster@rx.t-online.de with the specific details of how this has  directly impacted your business, including:

- What sending domain you are using
- # of T-online addresses on file
- Affirm all addresses are double opt-in (DOI is mandatory for delivery at T-online)
- Provide example email addresses that have complained to you about not receiving email
- How many emails do you send each month?
- Include an email header https://mxtoolbox.com/Public/Content/EmailHeaders/

I hope this helps gets your business marketing back on track and thank you so much for being part of the Klaviyo Community!

Hi Stephen, thank you for your answer. I think we had all requirements to sent mails successfully to our t-online receivers. I´m in contact with the Klaviyo support and I hope they will find an error our something else to optimize. I´m also in contact with the t-online support but they need some more information about the rebound error messages. In the backend I can only filter out if the mails are hard our soft bounced. 

Hey @Frank-DOGO 

Thank you so much for the update on working with Klaviyo Support. Usually with anything that is very specific to your account, Support is your best option for help as the team here in Community do not have access to your account information and can typically only give general advice and guidance. I am happy to hear that you are getting the help that you need!