[Mailman-Users] No Bounce Msgs, Not All Subscribers Receiving

[Easy]

I made another test on the 680 mailman subscribers.

In the past days the mailman list was used to send to the subscribers:
- NO bounce notices came back - NONE
- Many subscribers did NOT receive the message from Mailman

Tonight I sent directly to the 680 subscribers (not using mailman)
- I got all sorts of bounce notices
- Many more, if not all subscribers, received the email for the first 
time in a long long time.

Help is appreciated in sorting out how to get mailman to work well.

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Here are more details that may help sort this out. The indications are 
that the issue is within Cpanel (mailman & sending the message out) 
since the same subscribers are working through Topica and receive 
messages sent directly to them.
***
Not All Mailman Listserv Subcribers Receiving Daily Email*
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- The mailman list has some 600+ subscribers
- This mailman is part of the CPanel package - This was sent to CPanel 
forum - no response yet.
- This is a daily (M-F) listserv newsletter. The daily newsletter is 
only sent out by the moderator.
- The list is fairly stable in size with the usual unsubscriptions and 
subscriptions, a slight increase each month.
- Mailman had been running fine for months, until Aug/Sep when 
subscribers started sending messages saying the daily email was not 
arriving.
- Subscribers NOT receiving their email seem to be consistently the same 
subscribers. They are from many and varied ISPs including AOL and Earthlink.
- NOT ALL subscribers of any of the ISPs are being rejected. Many 
subscribers on ALL the ISPs are receiving the daily email just fine.
- Some 250 of the subscribers are now on a Topica listserv. They ALL 
receive the daily email each day. When the same message is sent to the 
mailman listserv some of these 250 subscribers do NOT receive the email.
- Emails sent directly to the subscribers, from the same email address 
that is used to send to the mailman listserv get through just fine, 
though email sent through mailman does not get to them.

How can this issue be further isolated and resolved?