United Internet (1&1, IONOS, GMX, web.de, mail.com)

United Internet is a German internet services company that operates several email brands including 1&1, IONOS, web.de, GMX, and Mail.com. European receivers tend to have high expectations for GDPR legal compliance, and in some cases email deliverability is aided when the sender is included in high profile "whitelists" and certification programs. You can use their postmaster site to investigate most issues, and their contact form to submit things for in-depth investigation. A more complete list of domains can be found at https://www.spamresource.com/2020/03/reference-webde-gmx-and-mailcom-domains.html

Links

Domains

Codes

501 · 550 · 552

501↑
501 Syntax error in parameters or arguments

Due to an incorrect configuration, email reception has been refused. Please contact your administrator who should correct the server configuration based on our recommendations and apply to us for the systems to be unblocked.

550↑
550 <example@example.com>: Reject due to SPF policy. The originating IP of the message is not permitted by the domain owner

The email was rejected, because it was sent from a server that is not included in the domain owner's SPF record. This usually indicates that the sender address has been spoofed and that the email is spam/phishing.

550 <example@example.com>: Syntax error in parameters or arguments

Due to an incorrect configuration, email reception has been refused. Please contact your administrator who should correct the server configuration based on their recommendations.

550 <example@example.com>: Requested action not taken: mailbox unavailable

The specified recipient does not exist on their systems or the recipient has not used their inbox for an extended period of time and the inbox has been temporarily disabled due to inactivity.

550 <example@example.com>: Bad DNS PTR resource record

Emails from your email server were rejected because the PTR Resource Record of your IP address does not follow their guidelines. Possible reasons for this can be: The PTR-RR states that the IP address was dynamically allocated or the PTR-RR is a generic standard entry of your provider. Please allocate an independent and fully qualified domain name to your email server.

550 <example@example.com>: Reject due to policy restrictions (header based)

Your message will be rejected by their system if: any of the technical information included in the email doesn't meet the standards, the email violates their guidelines or, it is not plausible, i.e.: when specifying the date. This is applicable to all emails with more than one of the following headers: BCC, CC, Date, From, Sender, Subject, To. In addition, the headers "Date, From, Sender, To" must be syntactically correct.

550 <example@example.com>: Reject due to policy restrictions (domain based)

The email has been rejected due to their current security policy. This may occur if the sender domain is known as a spam domain.

550 <example@example.com>: Reject due to policy restrictions (IP based)

If a high percentage of e-mails are classified as spam by their filters, all messages coming from the sender's IP address will be temporarily rejected for a specific time frame.

550 <example@example.com>: Requested mail action aborted: exceeded storage allocation, Quota exceeded

The potential recipient does not have sufficient free space in their inbox

550 <example@example.com>: IP address is block listed

Your IP is in one of the block lists used by Web.de

550 <example@example.com>: Invalid DNS MX or A/AAAA resource record

Your domain does not have an MX record or an A record that can receive replies or bounces, or the MX record is not resolvable.

552↑
552 Requested mail action aborted: exceeded storage allocation Quota exceeded.

We were unable to deliver your email because the potential recipient does not have sufficient free space in their inbox. Please use a different method to contact the person in question and inform them of the situation.