It would seem emails to your address are being bounced. Perhaps your 
mail server is a tad unreliable?
have you tried a different mail service?
Ian Zagorskih wrote:

>I've saw this message from majordomo many times during last year and had to 
>re-subscribe on NetBSD mailing lists again. Today i'v got seven such 
>messages.. Cannot say it makes me some real problems or excessive annoying me 
>but what is the purpose of this automaticall unsubscriptions? And what can i 
>do about it [if anything] to become "permanently subscribed".
>
>// wbr
>  
>