Im running an email server as part of a research study. We have subjects connecting and sending email from a variety of clients.
One function of this server is to communicate with said subjects - this means sending out regular emails. In the last few weeks it seems that nearly all of these emails are being marked as spam by the big providers (gmail, hotmail, yahoo, etc). As a result I'm looking into how to mark these messages as safe.
One suggestion that comes up from searching is to use DKIM. I can set this up and try it but it certainly won't be in place for all of our existing clients. If I implement it will it block all emails from our server that aren't setup to use it?
FWIW: Postfix, CentOS 6.4 x64