I also had problems mass emailing to my forum members. No errors, but no messages getting through either. I contacted dreamhost support and got this reply ...
"We have throttling put in place that restricts the number of
recipients in a single email to (I believe) 25. This is to ensure that
our servers are not overflooded with customer-generated messages."
If you have a managable number of members, split them into smaller groups (say 20 each) then send your message to each group. Thats what I'm going to try ...
Hold on, I found the "email posts or messages to combinations of groups" mod for phpbb ... lets you divide mass emails into smaller recipient chunks, amongst other things. That might work.