Is it from sending mass emails that look like spamming? I've never had a problem sending to Gmail but my personal server is only used by me and a few others for personal email.
We communicate via email exclusively with our already users and clients which in 99% of the cases contact us first (from same gmail address that mark our service as a spam), we don't have time to spam which anyway will damage our reputation.
The funny thing is that, All this happens mostly with traffic in our own continent in our own country (we are not even trying to get intercontinental clients)!
