Once you have setup your mail server, it is supposed to be a 'fire-and-forget' solution. And if you are the sole user, who hardly surfs on the internet and therefore never gets infected with anything, that might be true.
But for most people, there is always something that can and will happen to your system (ads, drive-by scripts, less scrupulous sites, etc.) and it takes just one thing to get flagged as a spammer by one of the many organisations, which patrol the internet in their fight against spam. Get on one of those black-lists and you will have to make an effort to clean up whatever mess occurred on your network, then apply to be taken off their list (which varies a lot) and wait until they take you off their list.
But you are still being observed for a few weeks to a month and if you manage to get on their list again in that period, it will take a lot more doing to be removed. Repeat offenders are not appreciated at all.
You might be able to look on the following sites to see if your web- and/or mail-host is on their blacklist:
The first 2 links are probably the most important ones to verify against. SpamHaus is a very important player and they are global. Not all organizations cover the same territories, so if you are on a blacklist of one of those, but you are never sending mail to that particular region normally, you still should get off that list, but you can continue to operate while you do this. In that case there is hardly any damage done. But you shouldn't expect your mail server to get a good reputation any time soon.
Private mail servers have almost no possibilities to get a good reputation anyway, just because they are privately managed.
