Do you use the newest version? There should be a 2.4 ready version where that problem has been solved. The only thing that might not work is unsubscribing.... (I went for an other newsletter system in the end - non silverstripe)
Regarding phplist: The code and interface look old and dated, but as far as I could see they are still updating it and adding stuff. I could not find another program, alas.
But one of the maintainers of the newsletter once send me an updated newsletter unsubscribe php with the remark that it was not 2.4 ready so I had to do that myself but somehow I had gone through so many problems with it already that I dropped it.
I can solve small problems with the modules, the saphire and cms code but I am not experienced enough in php to handle the bigger problems. Well that is what I use as an excuse anyway.
I SVN'd the latest version of newsletter module and i am still having the same problem. It errors trying to load a saved newsletter so that you can edit it or send it later. As of now I have to create a draft, type it all out, save it, and send it all without leaving that screen or I can never come back to it.