Spammers are the ultimate Sybil attackers. Setting up a useful SMTP server as an individual, and creating a new non-blacklisted identity as a spammer, are effectively the same task. The community of legitimate email providers has responded (quite effectively, and without too many false positives) by making this as expensive, difficult, and time-consuming as possible.
Emails from residential modems are not even worth scanning - they are practically guaranteed to be from botnets. Emails from commodity hosting providers are also pretty suspect, because they're very easy for spammers to get their hands on.
If you want your mail delivered, you need to send it from IP addresses that don't have those obvious red flags, don't have a reputation for sending spam any time in the distant past, and also have a long-term positive reputation for sending non-spam email.
In practical terms, you need to be in the professional mail server administration business full-time (be extremely careful to shut down abusive customers/tenants rapidly, never make a mistake that would let an attacker run an SMTP server on your network, etc.) or you need to pay someone who is, and who trusts you to cloak yourself in their reputation and not ruin it.
