1&1 is a large email and web hosting provider headquartered in Germany, but with a large presense throughout Europe and a significant showing in the US. They host mail using the domains mail.com, email.com, web.de, gmx.net as well as many other domains.
There are multiple reasons as to why this ISP may block a particular sender, IP address, domain name or mail stream. They do appear to attempt to block spam based on metrics at their disposal. High numbers of spam reports, fingerprinting of spammy content by a spam filtering service, blacklistings, and spamtrap hits could all be contributing factors to a blocking issue here. Meaning, having a good sender reputation, following best practices, and avoiding bad things like purchased lists and email append are all very important to maximize the chances of deliverability success when sending to 1&1's customers.
Their postmaster site contains a link to various bounce messages a sender can receive back, helping to break down the potential reason for any blocking or delays. These could be "policy"-based (i.e. related to spam issues or sender reputation), blacklist issues, authentication related, or due to other issues, so it's important to review that specific bounce response to best understand how to address. The bounce response itself is likely to contain a link to one of these postmaster pages. Always look for and follow that link as it is likely to provide good information.
They appear to participate in the Certified Senders Alliance, using the CSA whitelist to ensure "that your emails are evaluated with a lower spam probability and not forwarded to the user's Spam folder." Thus, participating in CSA's whitelist is likely to help improve deliverability to these domains.
Though each brand (Web.de, GMX and Mail.com) appears to offer its own postmaster site, I suspect that they all are just different branding wrapped around the same set of spam filtering rules and requirements.
When requesting unblocking, there's a separate form for each brand there as well (Web.de, GMX and Mail.com), so you might want to use the one that is specific to the domain(s) you're sending to, just in case it matters. I don't think it does, to be honest, but I'm not 100% sure. So, better safe than sorry.
When contacting this ISP, be sure to include as much detail as possible. I have always found them to be responsive, but I've heard complaints from others that they do not always respond.
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