You may be listed maliciously through one complaint of a client, or that of a competitor.
Probably not. Those DNSBLs (DNS blocklists) that are used by most major email providers are pretty rigorous, and use automated methods to determine offending mail systems. Typically, they won't block based on a personal recommendation, but rather on a characteristic of the mail server cofiguration, like whether that mail server allows "relaying" of mail by unrelated third parties (is "an open relay"). Getting removed from any of the big DNSBLs is quite simple: fix the mail server's configuration and have them test it again. It takes about 5 minutes.
Additionally, most major mail services do not use more than a couple of highly-reputable DNSBLs plus their own filters because, in the past, small, niche DNSBLs were unreliable, and often too aggressive in their blocking activities.
The bottom line is clear: If you are spamming (sending unsolicited email) then a good thing is being done when your message is blocked by whatever means. If you are truly sending out an opt-in bulk message, then your subscribers should be educated about how to allow your mail through their particular filters, and that's all it takes.
DNSBLs are easy to work with if your mail server is well-configured. And if it's not ... there is a price to pay, and rightly so.