1. You could check the following kb/posts
2. If the email is send through the newsletters module and there is no response for failure, then your mail server or ISP has blocked the email
3. There are many fixes since 3.5 editions, so I suggest that you should upgrade to the latest official release of Sitefinity - 3.7 SP4.
You could try manually to unsubscribe your user by using the code below
var dataManager =
var subscriberEmail =
"here type your email"
var gSubs = dataManager.GetSubscriberGroupsOfSubscriber(subscriberEmail);
ISubscriber subscriber = dataManager.GetSubscriber (subscriberEmail);
( subscriber !=
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