That's a VERY low number based on the amount of members you have. Ask your ISP for time to investigate and then give an initial response back to them. I would go the route of providing immediately a record of those on the list of 30 that are still members (some may have unsub'd). Give them a rough number of your list membership so they know up front how low this 30 complaints is to your list.
My gut is like others have said here most are spam rejects and not true issues. Contact the true members directly to get an explanation. Put in a time requirement to these 30 members for a response and indicate you will un-subscribe them at the end of that time period.
For any of the 30 member if your inquiry bounces as a 5xx or 5.x.x error unsub them immediately. Forward that to the ISP too. I wouldn't be surprised after doing this that your left with 10 or less. If so see if you can get the ISP to agree that you can handle the remainder independently.
Though I'm a Microsoft Exchange email guy I have found applications like phplist relaying thru my servers had sometimes left out headers that triggered a spam filter. I'll look at phplist and see if that might be true. I had a one way paging service rejecting emails that lacked reply-to:
header, too funny!
One last possibility is your messages seem spammy to the gateway rejecting it. There are tools to check a message's SPAM grade, you may wish to investigate them and tune your messages based on the tools recommendation(s). A few are listed here [emailexpert.org
PHPlist Help Forum post: Why are my messages being marked as spam
] was my 1st choice of the 124 hits found with this search string site:phplist.com email headers
Beyond this I assume the DNS (A, Pri-MX & Sec-MX) and rDNS (Pointer) are all perfect. Implementing Domain Keys
has been shown to clear up some spam false positives too.
Feel free to sticky the remaining spam rejects for my insight.