Numerous emails to Gmail are suddenly undeliverable - what is behind this?

What should you do if Gmail or other email providers refuse to accept your sent emails or if they are rejected by Gmail as undeliverable?

Who is this article for?

This article is relevant for business customers and private users who have integrated their own domain with us and use it to send e-mails. Private customers with an address do not have to worry about their spam reputation. We have already set all the necessary settings for you.

Why are my emails rejected?

If Gmail or other providers reject emails, this is called a bounce. This bounce can have several reasons, but the problem is basically Google's strict checking of various protocols.

Here is an example of a failed delivery because the message did not pass Gmail's DKIM check:

Bounce from Gmail

Datum: 22.07.2023 22:29 CEST
Betreff: Undelivered Mail Returned to Sender

This is the mail system at host I'm sorry to have to inform you that your message could not be delivered to one or more recipients. It's attached below.

For further assistance, please send mail to postmaster. If you do so, please include this problem report. You can delete your own text from the attached returned message.

The mail system <>: host [] said: 550-5.7.26 This mail is unauthenticated, which poses a security risk to the 550-5.7.26 sender and Gmail users, and has been blocked. The sender must 550-5.7.26 authenticate with at least one of SPF or DKIM. For this message, 550-5.7.26 DKIM checks did not pass and SPF check for [] did not 550-5.7.26 pass with ip: [].

This behaviour not only costs nerves but can also cause unnecessary costs. Because it doesn't matter whether it's marketing campaigns, customer support or general communication, e-mails are the preferred medium and an indispensable tool for your company.
What can you do if your e-mails are not delivered or even marked as spam?

To avoid such delivery errors and bounces (e.g. Undelivered Mail Returned to Sender) and to ensure that emails you send via your domain can successfully pass the spam filters of Google and other email providers, there are three helpful protocols: SPF, DKIM and DMARC.

What are SPF, DKIM, and DMARC?

SPF (Sender Policy Framework) is an authentication method designed to determine which mail servers are authorized to send emails on your behalf. It prevents spammers from exploiting your domain to send fraudulent emails.

DKIM (DomainKeys Identified Mail) is another authentication method that ensures the integrity of an email message. It uses a digital key linked to the email message to ensure it hasn't been altered during transmission.

DMARC (Domain-based Message Authentication, Reporting & Conformance) is a protocol that combines SPF and DKIM and provides additional guidelines for handling unauthenticated emails. It also offers reporting features that allow domain owners to receive feedback on the delivery of their emails.

Why are these protocols so important?

These three protocols are core elements of email authentication and play a crucial role in detecting and preventing spam and phishing attempts. As a result, they are closely tied to your domain's spam reputation and directly influence the likelihood of your emails being classified as spam or being rejected.

  1. Protection against Phishing and Spoofing: Cybercriminals often use fake email addresses to send fraudulent emails. Implementing these protocols ensures that only legitimate emails from your domain are sent.
  2. Improved Email Deliverability: Email Service Providers (ESPs) check these authentication protocols to determine if an email is legitimate. If your emails pass these checks, they are more likely to land in the recipient's inbox and not the spam folder.
  3. Increased Transparency: With DMARC reports, you can see exactly who is sending emails on your behalf, whether they were successfully delivered, and if they were authenticated.
  4. Avoiding Bounces: Emails flagged as spam or rejected for other reasons can lead to bounces. This can harm your domain's reputation and affect the deliverability of future emails.


For businesses, the correct implementation of SPF, DKIM, and DMARC can mean the difference between the success and failure of an email campaign. Some of the advantages include:

  • Increased Deliverability: Your marketing and transactional emails are more likely to reach your customers' inboxes.
  • Protection of Brand Reputation: By preventing phishing attacks and ensuring that only legitimate emails from your domain are sent, you safeguard your brand's reputation.
  • Enhanced Customer Trust: Customers are more inclined to respond to emails when they are confident they come from a legitimate source.
  • Cost and Time Savings: Businesses can save money and time by reducing bounces and increasing deliverability.

Act Now to Improve Reputation

By implementing SPF, DKIM, and DMARC, businesses can not only enhance their email deliverability and spam reputation but also protect their brand, strengthen customer trust, and ultimately maximize their ROI.

If you need more information or assistance in implementing these protocols, take a look at our detailed article on the topic: SPF, DKIM, and DMARC: How to improve spam reputation and avoid Bounces.

Check your configuration

You can check which protocols and settings you need to adjust with these tools, for example:

  1. - DMARC Check Tool
  2. - Email Communications Security Assessment (MECSA)
  3. - DMARC, DKIM & SPF Test

Example of - missing DMARC configuration

Beispiel - fehlender DMARC-Eintrag

Implementing and adapting configurations

If you receive error messages during the check by the tools mentioned above, additional settings are probably necessary for the DNS records of your domain provider through which you manage your domain and send e-mails.

If you need support or more information on the implementation of these protocols, take a look at our detailed article on the topic: SPF, DKIM and DMARC: Improve spam reputation and avoid bounces ->.

What is described in this article?

In addition to general information about the protocols, parameters and DMARC reports, you will find the following instructions with example configurations:

  • Set the SPF record and the parameters as TXT records in the DNS settings of your provider.
  • Set the DKIM records as CNAME records in the DNS settings of your provider.
  • Setting the DMARC record and the parameters as TXT records in the DNS settings of your provider.