If you plan to make a complaint to your email provider, you can share this page with them in order to explain the problem. More background on the problem is provided at: Email domains that you should not use for your Cultivariable account
To complain to your email provider, the address is usually email@example.com, so, for example, if you are firstname.lastname@example.org, you should complain to email@example.com.
For the email provider:
Cultivariable emails to our mutual customer are being rejected because you use a blacklist service called UCEPROTECT. The Cultivariable mail server is located on the Ionos ARN, which is routinely blacklisted at UCEPROTECT level 3. In addition, the server is located on a subnet that is frequently blacklisted at UCEPROTECT level 2. The Cultivariable server itself has never been blacklisted. These are sweeping blacklists that include a ridiculous amount of collateral damage and are, in the most charitable interpretation, meant to be used as one factor in a multi-part algorithm, but, for some reason, your company actually rejects emails based on these lists, apparently as a single factor.
The Cultivariable mail server is mail.centralcoastdata.net (126.96.36.199). If you can whitelist this address, the customer will receive our emails again. The better practice would be to stop using UCEPROTECT entirely, since you are probably dropping lots of other legitimate email.
Please contact firstname.lastname@example.org if you need additional information.