Residential IPs are generally proactively added to RBLs by the ISPs themselves as a form of spam protection. Before they started doing that it was basically whack-a-spammer all day every day for them.

you can also call your isp and get port 25 unblocked, gp's experience hasn't been my experience

I suppose you can, but in my experience, I think you'll find that people proactively find the dynamic ranges of IPs in eyeball networks and make it very difficult to get mail accepted. If you want to send email from your own server, you will either have to get a 'business/static IP' account or use an IP in a block where you can both control the RDNS and servers are expected to be. i.e. a VPS provider or colo IP range.

On a related note, what if I want to run a mail server but only for incoming emails? As in I want to buy a domain and create basically a sinkhole email address and create email addresses like [email protected]

I never need to reply from that email. I just need to be able to read emails I get. Should be much easier, right?

That should be pretty easy. It mostly depends on the exact configuration of the networking where your mail host is and your ability to correctly lock it down so it's not a spam machine.

Receiving email is pretty straight and forward, filtering it can be a little tougher but should be fine.