Hi Andrew,
You may or may not know, but I figured I would offer up a thought on this. If you got the error from Gmail stating something like "due to complaints", it is quite a bit misleading. I have seen Google do this with all of my web hosting infrastructure just because of the formatting of email list traffic or the amount of connections made to their servers. I have spent more time than I care to recount dealing with and learning how to get mail delivered in a large enterprise environment.
If I can help you out in anyway, please don't hesitate to ask.
Good luck,
Billy
-----Original Message-----
From: Andrew Hodgson