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(Write here your question/remark)Please help us help you and fill where relevant: -
Your LimeSurvey version: LimeSurvey Community Edition Version 6.15.20
Own server or LimeSurvey hosting: Own server
Survey theme/template:  -
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Hello,

could you please assist with the following issue?
LimeSurvey is installed via Docker and runs on a local server of our working group.

The currently active survey sends registration and confirmation emails via SMTP.
Yesterday, several participants did not receive these emails, although:

1. the participants were successfully added in LimeSurvey,
2. personal invitation links were generated,
3. the email status in LimeSurvey was reported as “OK”.

Participants from other domains did not report any delivery problems.

Given that our working group collaborates with medical institutions, identifying the cause of this issue is critical. The problem was identified only after the affected participants contacted us directly via separate emails.

Could you please advise on the possible causes of this behavior, including SMTP configuration, recipient-side filtering, or LimeSurvey-related limitations?

Please let me know if you require any additional information.

While it can't be discarded 100%, based on your description the problems are most likely not directly related to Limesurvey.

Most respondents do not have these problems, the issues are with specific domains. So the setup in Limesurvey and the SMTP server seem to work.

Sending mass emails is a science in itself. Trouble shooting is not easy, especially when the problem is with the receiving mail servers, as it seems, as this is no general issue, but an issue with specific mail servers on the receiving end.

Most likely the server on the respondents end is considering your emails spam, which can have a variety of reasons:

• Email text and subject contains potentially "suspicious" keywords that trigger spam filters
• The IP of the SMTP server used to send the emails is on a black list, the server is in a "bad neighborhood", or from a provider that is known for a high ¨% of spam activity, etc.
• SMTP server that is not correctly configured. Today, many receiving mail servers refuse to receive emails from servers that do not follow the rules of best practice on how to configure an email server
• Too many wrong email addresses in the database for this email. Email providers often consider a source as suspicious when the send emails to a lot of non existing email accounts for their domain.
• etc.

Did you configure a bounce email? Some email servers "respond" with the reason, when they block emails to be delivered.

What is the SMTP server you are using? Something like Sendgrid? Or is it hosted by your organisation?

All in all, given that the system generally seems to send emails (many respondents seem to receive the emails just fine) I think we can ignore the configuration in LS or the configuration of the SMTP account. I think you will need to look into the configuration of the server, the neighborhood of the server, potential spam filters on the receiving end, etc.

Thanks for your involvement and time.The bounce email is now configured; previously, it was not.Some details about the survey setup:

• Software: LimeSurvey Community Edition, version 6.15.20
• Deployment: Installed on our own server
• SMTP encryption: StartTLS
• SMTP username: Same as the working email address

If the participant’s server environment restricts email delivery, do you think it would be possible to configure this from our side?

If the participant’s server environment restricts email delivery, do you think it would be possible to configure this from our side?

Despite providing a "clean" email server that follows the rules, is not in a bad neighborhood and clean Email content, there is very little you can do.

In my opinion, it all comes down to the SMTP server that you use. If the SMTP server is configured well and on a clean IP or not in a bad neighborhood (IP range with a lot of spam cases).

You would need to speak with the admin of the email server on the other side to understand what is going on.

We had a similar case some years ago, where we were sending email invites to employees of a company (our client was exactly that company, who had hired us to do this research with their employees). So we talked to their IT department to make sure that our mails will reach them.

Different companies have different approaches to spam / receiving mass emails. Some important things to consider:
1. Make sure that you have a good contact list with valid email addresses. Many email servers put you on a block list when you send a lot of emails to accounts that do not exist (e.g. old emails that do not exist anymore, typos, etc.)
2. Make sure to not sent all emails all at once, send them in different steps, small tranches.

but as I said: Sending mass emails is an art in itself. You are always on the verge to be considered spam. If you send too many emails, users might report you as spam. If the text is using "fishy" language - spam filter. If you send emails from a server located in a "bad neighborhood" - spam filter. if you have a lot of emails bouncing - spam filter. If you have a lot of links in your email - spam filter.

It is tricky. Email is almost dead because of all the spam.