User unable to unsubscribe

Hello,

I'm managing mailing-list with ezmlm. 
When users want to unsubscribe, they received the following message: 

>I'm sorry, I've been unable to carry out your request,
>since the address
> 
>   user@address
> 
>was not on the xxx mailing list when I received
>your request and is not a subscriber of this list.

But when I check if the user is in the list, he is!!
I can unsubscribe manually but not with ezmlm-manage

Did you have some idea to solve the problem?

Thanks