Poor e-mail/internet service

I have been a Vox Telecom customer for more than 10 years, and encountered problems with the service from time to time, but never to the extent where I had to resort to a complaint on this forum. For almost a year now I have had a problem with e-mails bouncing back/not getting delivered. (I actually received mails in this regard, informing me that the e-mail could not be delivered) I have spoken to their technical department on a number of occasions, each time getting a different excuse, from 'line flapping' to my mail address being blacklisted?? Even an excuse that I cc'd too many recipients on a particular mail? Bottom line, the problem never get addressed, as it seems that the technicians are not competent/able to sort out the problem. I even recently asked for a senior manager/- staff member, to contact me to get the matter resolved, but without any response, not even upon my request that somebody phone me back. I am at a point where I'm seriously considering, moving my business elsewhere to another service provider.