Spontaneous unsubscription

From time to time i get the information via e-mail:

...@web.de has unsubscribed
Reason given:
"Jump off" used by subscriber, reason not requested

in the config i have:

if a user should immediately be unsubscribed, when using their personal URL, instead of

the default way, which will ask them for a reason, set this to 1

define("UNSUBSCRIBE_JUMPOFF",0);

In my newsletters i don’t have any unsubscribtion or forward links. i wish that recipients write me back to unsubscribe.

the topic in the old forum couldn’t help. the ip seems to be fine.

any idea what the problem can be?
thanx in advance!