FAQ

We want to give our customers the information they need to get the most from our STACKIT MailOut product. This FAQ section answers common questions. This helps you quickly find solutions and improve your experience. We encourage you to check these FAQ before contacting our support team, as you might find your answer here.

• General

  Expand all

  Why aren't my emails always delivered right away? (greylisting)

  If your transactional emails (such as order confirmations, password resets, or verification codes) are delayed in reaching the recipient, †this is very often due to what is known as greylisting by the recipient’s email provider. It is a widely used anti-spam measure employed by recipient mail servers. If a receiving server receives an email from a sender IP address that it does not yet recognize (sometimes in combination with the sender and recipient addresses), it initially rejects delivery with a temporary error. For more information, see RFC6647.

  This is not an error or a sign of a delivery problem, but rather normal behavior.

  The reason for this is that reputable mail servers adhere to the SMTP standard and retry delivery after a waiting period. Many spammers do not do this and give up after the first attempt.

  Examples of greylisting responses from receiving mail servers:

  • 450 Greylisted, please try again later
  • 451 Greylisting in action, please come back later
  • 450 Now greylisted - please try again in five minutes.,
  • 451 Service unavailable - try again later
  • 450 Action request delayed, please retry
  • 451 Temporary Local Problem. Please try again later

  How does greylisting work?

  • First delivery attempt: The receiving mail server is not yet familiar with our STACKIT MailOut service and rejects the email on the first delivery attempt.
  • Automatic pause: STACKIT MailOut waits for a short period of 5 minutes for the second delivery attempt.
  • Second delivery attempt: STACKIT MailOut attempts delivery again. Only now does the recipient’s mail server accept the email and deliver it.

  For the recipient, this means that an email arriving from a specific sender’s infrastructure for the first time is delayed by a certain amount of minutes.

  To ensure that your emails arrive in the recipient’s inbox immediately, without delay, you should contact the IT administrator of the recipient’s email server and ask them to add your sending domain to the whitelist or allowlist.

  How does MailOut handle temporary delivery errors? (Retry mechanism)

  If STACKIT MailOut encounters a temporary error (4xx) during a delivery attempt – such as due to greylisting or a temporarily unreachable recipient — the delivery will automatically be retried at later times. The following schedule applies:

  • First retry: after 5 minutes
  • Second retry: after another 10 minutes
  • Third retry: after another 20 minutes
  • Fourth retry: after another 40 minutes
  • Starting with the fifth retry: at 60-minute intervals

  These delivery attempts continue until the email has either been successfully delivered or the message has spent a total of 96 hours (4 days) in the queue. If it cannot be delivered within this period, it is removed from the queue.

  This mechanism ensures that short-term disruptions – such as greylisting during the initial contact, brief unavailability of the receiving server, or temporary rejection due to server load – do not jeopardize delivery, without keeping emails in the queue indefinitely in the event of an error.

  How to extend the SPF DNS record to use multiple email services?

  If you want to use other email services in addition to MailOut, you have to administer your SPF DNS record to add all services that you want to use.

  Instead of creating an additional SPF DNS record, you need to add a new “include:” attribute in your existing SPF DNS record of type TXT. If you want to use STACKIT MailOut and Google Mail to deliver email messages, your SPF record must look like:

  v=spf1 include:adbcca95f52e36bc.mailout.eu01.dev.stackit.cloud include:_spf.google.com ~all

  How can I increase the sending quota for my sending domain?

  Please create a ticket on our STACKIT service desk.

  Please include the following information in your ticket:

  • The sending domain to increase the quota for.
  • Estimated email volume per hour and per day.
  • The type of messages you want to sent (password reset emails, newsletters, invoices, etc.)
  • What specific measures do you take to ensure a consistently high email delivery reputation? (e.g., How were the recipients’ email addresses collected (Double Opt-In)? How do you handle hard bounces and maintain mailing list hygiene (Unsubscribe Link)?)

  The sending quota limits apply per sending domain.