Please help us help you and fill where relevant:
Your LimeSurvey version: LimeSurvey Community Edition .  Version 5.4.8+221024
Own server or LimeSurvey hosting: Own server
Survey theme/template: A fork of the Fruitty theme
==================
I set up 2 surveys ..1 with 75 participants and the other with 15.  The smaller one went out just fine with no issues.   The larger one with 75 go out just fine but a few minutes later, the Email status show OptOut.   The company uses spam filtering and already have whitelisted my emil address.  The IT person there shows that 
a) they got the email
b) then the spam filter quarantee shows duplicate messages 

Manual does not say what would cause these to "auto opt-out" so it makes it more difficult for me to figure out what's going on.

Thank you

jp
 

Surely a link checker …

If you have an automatic system that
1. Check the link in email.
2. Then when goes to the link : if this system click on the button (?) : it can happen.

In general 1 happen, rarely 2. Maybe you need to check your email spam protection system.

Please : report the issue, and maybe remove the {OPTOUTURL}

We are experiencing the same issue and have now observed it with multiple customers using different corporate mail systems.
It seems to be happening more frequently, likely caused by email security or anti-phishing tools that automatically follow the opt-out links in survey invitations and even trigger the confirmation click.

We are currently trying to identify which specific security products (e.g., Microsoft Defender Safe Links, Proofpoint, Mimecast, Barracuda, etc.) are responsible in each environment.
It would be very helpful if other users could share which email security tools or filters they have confirmed as the cause on their side.

We have also created a detailed bug report to document this behavior and suggest a technical solution based on a two-click confirmation procedure:
bugs.limesurvey.org/view.php?id=20317

The problem is becoming increasingly relevant, as automated opt-outs make it impossible for some clients to conduct surveys reliably.

Still using email? Who still reads emails?