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Boost the deliverability of your email messages.

Find out how this can best be achieved from Gernot, Ellen, Stephan and Peter.


What is spam?

Spam (the unofficial name for unsolicited bulk email (UBE)) refers to bulk emails or advertisement emails that are disseminated throughout the Internet. They land unsolicited in millions of electronic inboxes. Most spam emails are commercially motivated and are categorized as follows:

  • Commercial email spam
  • Chain letters, virus warnings, hoaxes
  • Emails sent by viruses
  • Phishing emails
  • Delivery (delivery rate)

    The delivery rate is the percentage of successfully sent emails (defined as receipt of the email in the inbox of the recipient) compared to the total number of emails sent. Bounces, meaning undeliverable emails, are not included. The delivery rate is, among other things, an indicator of the quality of the distribution list. To keep the delivery rate consistently high, a process known as bounce handling is needed. This process is also integrated in Retarus’ global suppression solution. Email addresses that cannot be delivered to are automatically removed from the distribution list.

    Reputation/sender score

    Scores are calculated as a moving average over a period of 30 days and represent the ranking of an IP address compared to other IP addresses. It is similar to a percentile rank. The closer to 0 the score is, the worse it is. The closer to 100
    the score is, the more effective the sender.

    Complaints: This is calculated by dividing the number of complaints by the number of received emails. It also takes into account the number of complaints the IP address receives compared to other IPs.

    Volume: The dispatch volume alone is not an indicator of whether a sender has a good or bad reputation. However, it is an important part of the general algorithm. For example, an IP address that sends 100 emails and receives 99 complaints is ranked as questionable. In contrast, an IP address that sends 100,000 emails and receives 99 complaints is ranked as having a good reputation. Thus, the volume always depends on other index values.

    External reputation: This number indicates how a sender’s IP address compares with other IP addresses also listed on numerous external blacklists and whitelists.

    Unknown contacts: The ratio of an IP address’ unknown contacts to other addresses is taken directly from the incoming SMTP logins of the involved ISPs. It measures how often an IP address attempts to send an email to a recipient that does not exist.

    Rejected emails: This measures how often sent emails result in a soft or hard bounce as compared to other IP addresses.

    Accepted emails: This number represents how many emails are accepted by the ISPs and forwarded to the recipients. The number is comprised of all sent emails minus the number of rejected emails.

    Ratio of accepted emails: The number of accepted emails compared to the total number of sent emails. To obtain this ratio, the number of read emails is divided by number of sent emails.

    Ratio of unknown contacts: The number of unknown contacts or invalid email addresses is compared to the number of sent emails.

    SPF record

    The Sender Policy Framework (SPF), formerly known as Sender Permitted From, is a method designed to prevent the falsification of email sender addresses. It was created as a way to prevent spam. SPF is a record in which the owner of a domain listed in the Domain Name System specifies which computers are allowed to send emails for this domain. The administrator of a domain saves a TXT resource record (RFC 7208 has rendered the SPF resource record obsolete) in the DNS zone. These resource records contain the IP addresses of the mail transfer agents (MTA) that are permitted to send emails for that particular domain. Using the domain specified in the MAIL FROM and HELO fields, the recipient verifies whether the sender is authorized to send the email. The recipient calls up the SPF information via the Domain Name System for the specified domain and compares the IP address of the sending MTA with the permitted addresses. If the IP addresses match, the sender is authentic. If not, the email can be discarded.


    DomainKeys is an identification protocol that verifies the authenticity of email senders. It was designed to help prevent and stop unwanted emails such as spam or phishing emails. DomainKeys are based on asymmetric encryption. The email is given a digital signature that the recipient server can verify using the public key, which is available in the Domain Name System (DNS) of the domain. If verification fails, the recipient mail transfer agent (MTA) or the recipient application program can refuse the email or sort it out from the others.

    Other definitions in the transactional world of email

    Inbox placement rate
    Deliverability rate minus the number of emails that landed in a spam or junk email folder

    Soft bounce
    Emails sent to existing addresses that could not be initially delivered

    Hard bounce
    Emails that could never be delivered

    Opening rate
    Number of openings divided by the number of delivered emails

    Unique click rate
    Number of single clicks in an email by a user (no repeated clicks) divided by the number of delivered emails

    Total click rate
    Number of all clicks in an email divided by the number of delivered emails

    Click to open rate
    Unique click rate divided by the opening rate

    Conversion rate
    The number of targeted actions divided by the effective click rate

    Bounce rate
    The number of bounces divided by the number of delivered emails

    Risks and pitfalls

    A current study conducted by Return Path, an email intelligence company, determined that 20 percent of sent and desired emails do not reach their intended recipients. Undelivered emails hurt a company in two ways:

  • There is a correlation between a marketing campaign with a smaller reach and a loss of revenue. The revenue potential that lands in spam folders each year in Germany is estimated to equal approximately 3 billion euros.
  • Yet companies incur more than just financial loss when emails are mistakenly classified as spam. The reputation of the company and the brand also suffers when sender addresses appear on blacklists and the company receives a negative rating as an email sender.
  • Causes of spam classification

    There are many reasons an email is classified as spam: spam traps, cold IPs, the design of the email, and reputation scores. It is natural that users want to use security software to protect their email inboxes and computers from attacks that are sent through the Internet. It is advisable to consider a customer’s security needs when planning email marketing activities.


  • Directory of IP addresses and domains that are classified as negative because they send unwanted emails
  • Emails sent via these IP addresses are intercepted and blocked
  • Large email providers typically have multiple blacklists


  • Protects the user from undesired communications


  • Desired emails are placed on the blacklist, e.g. if a blacklist blocks an entire domain even though only one subdomain is sending spam emails
  • The sender suffers loss of revenue and damage to the company’s image; campaigns that are sent via this IP address are no longer delivered
  • Time-consuming delisting. It is possible to be deleted from a blacklist. However, if the sender commits multiple offenses, he will eventually be placed permanently on the blacklist
  • Unfair business practices


  • Rejection of an unknown email address and/or IP address on the first attempt at delivery
  • A subsequent second attempt is usually successful


  • Effective spam prevention: many spammers give up after the first try. Real email servers typically try to deliver a second time
  • After successful email delivery, the combination of sender, recipient, and email server is entered in the whitelist and subsequent emails are delivered
  • Greylisting impacts the quality of the email. Thus, there is no automatic opt out


  • With greylisting, an unwanted email can be delivered a few minutes or even a few hours later
  • As soon as the first attempt is made, some email server programs generate a temporary delivery report for the sender, which can be interpreted as failed delivery in some cases
  • Using IP tools could negatively impact the success of email delivery
  • Guidelines for good deliverability

    Deliverability is one of the most important keys to professional email marketing. The quality of the systems used by companies to send email campaigns is important. They must be able to maintain high deliverability rates and be expandable.

    Warming-up unknown IPs

    The reputation of IP addresses is primarily based on historical sending patterns and volumes. An IP address that sends consistent volumes of emails over a long period of time typically means it is a highly reputable address. Companies that use dedicated IP addresses maintain their reputation as a sender by sending consistent and predictable volumes of emails. Therefore, Retarus recommends the following when warming-up a dedicated and unknown IP address:

  • Duration of warm-up process: 6–8 weeks
  • Start by sending a low volume of emails:
    » Max. 1,000 emails on day 1
    » Double the volume based on the
    previous day
    » Start with controlled dispatch speed
    » Slowly increase the volume, number of
    recipients, dispatch speed
    • Cleanup the user list in parallel
    • Use all available IPs and domains regularly
    so they remain known and are considered
    • Do not implement a sudden and extreme
    • Keep the risk of blacklisting to a minimum
    (beware of spam traps and affiliate campaigns)
    • During the warm-up, dispatch attractive
    campaigns, not strategically important ones
    • Focus the initial dispatch on particularly active
    and interested customers