Alright, well they got back to me with this explanation:
Your IP has been blocked because of spam issues or because your ISP indicates that it is dynamically assigned
I am using a DigitalOcean VPS, which they say is a static IP address.
So, the only other possibility is the “spam issues”.
In that case, there are the following possibilities:
- The IP I am assigned was previously used by a spammer, and blocked by Verizon (but, apparently never submitted to Spamhaus).
- My server is currently sending out spam. I know MiaB uses SpamAssassin to protect against incoming spam, but what about outgoing? Various online tools tell me that my server is not acting as an open relay, and only authenticated users can send email through my server. So, the only thing I can think of is that perhaps one of my users’ accounts has been compromised. Is there a rate-limit configured for outgoing email, or another tool to detect compromised SMTP accounts?