i'm having the same issue. i'm using dada mail, which uses sendmail on the backend to do its sending.
my announcement list, which is about 3000 emails, was sent out yesterday morning. i have about 10 of my own emails on the list to make sure it arrives in different places-- one arrived on time. two arrived a few hours after they were sent, but the others never arrived. some of my friends on the list never got the email either.
i've had an open support request (with a requested callback) open for about 24 hours now.
the annoying thing is, if i do have to re-send this mailing, i'm most likely going to have to double-send the mail to some people-- unless there is an error log or some indicator of who actually received the emails.
i've used this process successfully in the past, so i'm assuming its due to the DOS attacks they had yesterday, or some email reconfiguration issue.. hopefully someone gets back to me soon and figures this out!!!