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
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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}