[Mailman-Users] Not all subscribers receive emails

[Matthias Rank]

Hi everybody.
I have a problem with Mailman. When I send an email to a list this won't 
be any problem for very little lists up to a size of about 15 
subscribers. But if the list grows there are more and more people who 
don't receive emails and produce bounces like "testlist: a at b.de bounce 
score: 1.0" or "testlist: a at b.de has stale bounce info, resetting". The 
amazing is that there is no obvious system which persons do not receive 
emails, especially these are not always the same people and they are 
member of different email providers. Furthermore there are some (few) 
people who do hardly receive any email sent with Mailman although their 
email-accounts are okay.
I'm using plesk, so any other mta than qmail won't work, if this might 
be the problem.
I'm very confused and would be very glad about help.
Regards, Matthias