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Serial mails do not arrive

When sending serial mails, it happens from time to time that single or all emails do not reach the recipient. This guide will help you to find the error.

The general procedure for sending serial mail is as follows:

  1. The serial mail dispatch is scheduled or manually triggered.
  2. SoSci Survey passes the serial mail to the mail server OUT for outgoing emails.
  3. The mail server OUT contacts the mail server IN, which should accept the mail.
  4. The mail server IN checks if it wants to accept the mail (if not, the process stops here).
  5. The mail server IN accepts the mail and transmits it to the mailbox.
  6. The mail is checked by spam filters and delivered to the mailbox belonging to the email address.
  7. The recipient picks up the email from the mailbox, reads it, and happily participates in the survey.

If the email does not arrive, it can be lost at any of these steps. And, of course, there is always the possibility of a technical error. After all, while emails are generally reliable, technology does not, in principle, guarantee delivery or that it will be delivered within a certain time.

If emails do not arrive, it makes sense to check the following sources of error.

Check dispatch

Did SoSci Survey send the email in the first place? To check, open the address entry in the address list that did not receive an email to edit it (edit address entry). At the very bottom of the address entry, you will find the Received serial mails section.

No dispatch entered

Check if the missing serial mail is listed there. If not, check if this has affected multiple recipients. To do this, click on Prepare sending (envelope icon) for the affected serial mail and check the Sending statistics for how often the serial mail was sent.

Error source manual sending: Did several address entries not receive the serial mail? Did you send the serial mail manually via Send invitations? And did you do this not time-controlled, but via Send immediately? Then make sure that the sending is really complete. SoSci Survey sends packets of 100 emails at a time. If an error occurred, you have to continue manually. Otherwise, it will continue automatically.

Error source mailSchedule(): Did you prepare the dispatch in the questionnaire using mailSchedule()? Then it is possible that the person did not even get to the page with the mailSchedule() when filling out the questionnaire. It could also be that the command was not executed due to a PHP filter.

Explanation of subsequent address entry: Was the address entry after sending the serial mail?

Dispatch entered but unsuccessful

If the dispatch is registered, but the status icon is blinking in red-yellow, the delivery to the mail server OUT or from there to the mail server IN was unsuccessful. Read more about this below.

Email transmission unsuccessful

If SoSci Survey shows the status “invalid e-mail address” for an email (red-yellow flashing), then either the mail server OUT or the mail server IN did not accept the email. The mail server IN can either reject it directly or send a notification “Undelivered Mail Returned to Sender” after a while. SoSci Survey collects these notifications and then marks the unsuccessful sending as “invalid e-mail address”.

Note: The status will also be “invalid e-mail address” if there was another problem with the send.

Note: It also happens that the mail server IN accepts and delivers an email and then still sends an error message. For example, if forwarding to another mailbox configured by the recipient did not work. In such cases, an email can be delivered and the address entry can be marked as “e-mail address invalid” at the same time.

Mailserver OUT

You may have entered your own SMTP mail server (mail server OUT) in the project settings, so the emails are sent directly through your setup. Principally, this is a good idea, but two errors can occur.

Mailserver IN incorrectly configured: Test in the Project Settings in the Special tab whether a test mail can be sent with the settings defined for the SMTP server (button Test). It may not accept emails directly from SoSci Survey. The displayed error information will provide further guidance if the test is unsuccessful.

Mailserver IN Limit: Possibly, the SMTP server also limits the number of emails that may be sent per day (or per hour). When the limit is reached/exceeded, sending further emails is rejected.

Please note that some problems are also temporary. Maybe the SMTP server was in maintenance mode (rare) at the time of serial mail sending and therefore did not accept any emails.

Mail server IN

If the email was successfully accepted by the mail server OUT but could not be transmitted to the mail server IN, it may have the following causes:

Error source email address: Perhaps the specified email address is wrong. It happens quite often that instead of the correct a muster.mü or a is entered.

Error source email inbox deleted: It regularly happens that email addresses of companies and universities simply no longer exist because the person has left the respective institution.

Tip: Open an address entry for editing (see above). The Received serial mails section will not only show when and which serial mail was sent to the address. It also shows the personalized link which was sent. Copy the link and write an email to the respective mail address in your standard email program, including the personalized link. If the address was correct, the person has the link to participate. If the address was incorrect, you will get an error message with more details.

Failure source full mailbox: It happens regularly that email mailboxes are full. If a mailbox is “Over Quota” or “Quota Full”, no more emails can be delivered.

Error Source Blacklist: Not everyone is happy to receive invitations to online surveys, especially if their email address is on some obscure address list where someone has collected email addresses, so the person keeps getting pestered with completely inappropriate invitations. In any case, complaints about invitation mails, sending to invalid email addresses, or even sending a large number of emails can cause a mail server IN to put the mail server OUT on a blacklist. The mail server OUT can then no longer send emails to the mail server IN. This may affect all survey projects on the survey server. If the mail server IN is, e.g., the one of GMX, or Googlemail, this can lead to massive restrictions in sending serial mails. The block entries are usually deleted automatically after a while.

Error source SPF entry: If the mail server OUT does not match the sender address, it looks to the mail server IN as if someone wanted to forge the sender. Please read the instructions Serial Mails and SPF Entries.

Email sent but not received

One step “softer” than the block list is the spam filter. However, it is unforgiving over a longer time. A spam filter usually uses different criteria – among them, the mail server OUT from which an email comes, the sender address, but also the content of the email. If serial mail ends up in the spam filter, the addressees usually do not find them.

Error source spam filter: If you are informed about missing serial mails, you can ask the person to have a look at the spam filter. By the way, in rare cases, spam filters delete emails without informing the addressees.

If your own emails get caught in the spam filter more often, the following measures can help:

  • Send emails via your own mail server OUT (SMTP server) instead of the mail server of the survey server, see Use SMTP server.
  • Use a different return address, see Serial Mails and SPF Entries.
  • Change the email's wording – avoid words and phrases that you would expect to find in spam emails.
  • If you have created or added something to the HTML part of the serial mail yourself, do not use images from external sources there (https://...).
  • Do not send spam. Ensure that you only send emails to email addresses that have agreed to receive them (this may even be required by law!) or that you can assume with a high probability to perceive the invitation positively.

Error source technology: It can happen that a mail server is defective or incorrectly configured. Sometimes, temporary problems occur that are fixed after a while. This can cause emails to be lost.

Advanced Troubleshooting

Mail servers usually record when they have contacted a mail server IN and whether this has accepted the email. The operator of the mail server OUT has access to the log files. So if you send, e.g.,, emails via the standard mail server (not an own SMTP server), then SoSci Survey GmbH could have a look into the logfiles in important and justified cases.

The log files, for example, look like the following. Scroll a bit to the right to read the complete entries.

Apr 9 21:26:31 s2survey postfix/pickup[8289]: 0ED8E32001F: uid=33 from=
Apr 9 21:26:31 s2survey postfix/cleanup[12430]: 0ED8E32001F: message-id=
Apr 9 21:26:31 s2survey postfix/qmgr[2214]: 0ED8E32001F: from=, size=848, nrcpt=2 (queue active)
Apr 9 21:26:38 s2survey postfix/smtp[12433]: 0ED8E32001F: to=,[]:25, delay=7.1, delays=0.08/0.03/6.1/0.88, dsn=2.0.0, status=sent (250 2.0.0 Ok: queued as 47AB865D11)

As you see, it is first recorded that the mail server OUT on s2survey has an email coming from SoSci Survey (“pickup”). For this email, SoSci Survey has entered the technical (!) sender, which actually means, supplemented by the identifier of the address entry (1075407) and the serial mail (15001), so that returns can be correctly assigned. The email also gets a unique identifier (0ED8E32001F) from the mail server OUT, which appears in all further lines which refer to the mail transmission.

In the next step (“cleanup”) the email is checked for formal criteria. It also gets another unique identifier (, which is then also written into the email and transmitted to the mail server IN.

In the third step ('qmgr'), the mail server OUT adds the email to the queue for sending.

In the fourth step, the mail server OUT (s2survey) now contacts the mail server IN, which is responsible for receiving the email. The email was sent to The mail server is responsible for this email address. Two pieces of information are relevant here: First, that the email was accepted by the mail server IN (status=sent). If this were not the case, an error would be displayed here (e.g., status=bounced with further details). Secondly, there is an identifier under which the mail server IN processes the email (47AB865D11).

The status often already explains why the email could not be delivered. If it was delivered, the operator of the mail server IN can see why the email did not arrive in the addressee's mailbox from the timestamp (at the very front, Apr 9 21:26:38), the email identifier from step 2 ( and the processing number from step 4 (47AB865D11) in its own logfiles.

People and email clients

The most frequent source of error after the spam filter is the recipient of the serial mail.

Error source recipients: Addressee:s quickly move an email they can't directly classify to “irrelevant” or “spam”, or they accidentally press the Del key.

Choose a unique subject and ensure people understand the email's content immediately.

Error source motivation: If there simply is no time or desire to participate in a survey, the email is ignored or deleted. When, after 3 days, a query arrives asking whether one received the initial email, it is often already forgotten (“No, there was nothing”).

Error source mail program: Sometimes email programs like Outlook or Thunderbird and Webmailer like GMX,, or Googlemail have minor problems. An email is sorted into an old conversation it has nothing to do with (it is then quasi-untraceable), or it is displayed with the data of another email in the message list and perhaps only found by chance.

Lack of motivation, human operating errors, and technical problems are disruptions that must be reckoned with in any survey. The first two aspects concern not only online surveys but also classic paper-pencil surveys.

en/survey/mailing-lost.txt · Last modified: 13.01.2023 11:53 by schuermann.linda
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