A while ago, my dear colleague Mattijs came with an interesting option in BIND. Response zones. One can create custom “zones” and enforce a policy on that. I never worked with it before, so I had no clue at all what to expect from it. Mattijs told me how to configure it (see below for an example) and offered to slave his RPZ policy-domains. All of a sudden I was no longer getting a lot of ADS/SPAM and other things....
If you go looking for a usable webmail application, then you might end up with Open-Xchange (OX for short). Some larger ISP’s are using OX as their webmail application for customers. It has a multitude of options available, using multiple email accounts, caldav/carddav included (not externally (yet?)) etc. There are commercial options available for these ISP’s, but also for smaller resellers etc. But, there is also the community edition available. Which is the installation you can run for free on your machine(s)....
For many System Administrators that have public facing Mailservers, it is an ongoing battle.. SPAM. Since there is money to make, it will never ever go away, but we can try to mitigate this. Introduction on my usage of anti-spam products For many moons I have used the SpamAssassin product in various forms, simply as a client to check every email on delivery, as daemon where multiple servers check one instance, as part of MailScanner where a single (replicated) database was responsible for storing all bits and pieces combined with local additional rules....