You’re probably getting the mailer daemon errors because they’re using fake email addresses—not because they’ve blocked your IP.
You can easily find someone’s IP from their email. After all, they have to designate the origin (just like you leave your return address on an envelope). Here is a list of ways on some major email services. If you use something that’s not in that list, the basic idea is to look for some way to show “details” or “header” (or just google for your specific program/website).
I actually have never even thought about blocking an IP address, never had a need to, so I don’t know how possible it is. But if it’s possible, then I’m sure it’s an easy process.
However, IP addresses have tendencies to change. Either just as the way that their connection is set up, or specifically done by the user, or they can spoof the IP address (Make it appear to come from a different one).
I would say to just get in contact with your email provider and ask them how to handle it.
On a side note, what kind of email is it? Is it like, somebody sending you (specifically) nasty letters? Or is it random gibberish? Or is it annoying spam selling you bigger penises and shinier diamonds?