You are of course correct AndyII, what I should have said is that I have only ever seen it when the transport is disabled.
More correctly it means that the call to the mail function returned an error. From what you describe, I'd check how much memory PHP has, and possibly up it a bit? Could be a memory leak or incremental memory use on the repeated calls is using up all the available mem, at which point it can't instantiate the next piece of mail?
I have also heard of it with badly formed email addresses, which would only make sense it it happened at the same or similar place every time, but that could also be a mem problem.
and finally, if the mail server (even your own), has a mails per hour limit set, it could be refusing the mail?
Rgds,
of course your right Chuckl, thing is I also run a different newsletter script and send out couple thousand emails using the php mail function, that doesnt have any problems so I wondering if its a zen thing, this is the difference between my 1st mailing and the one that failed, 200 more subscribers and the weight of the letter was 2kb more. going to check some logs and see if I find any answers :)