How do you contact an ISP about a block that doesn’t have a postmaster page? While there’s no one answer, I do have some suggestions.
Start by contacting the postmaster@ or abuse@ addresses. For smaller ISPs, the same people handling outbound abuse are the people handling inbound filtering.
When you contact them have the following:

• What IPs you’re sending from.
• What the rejection message is (or if it’s not a rejection message, that the mail is undelivered or going to bulk).
• The recipient you’re sending to.
• The type of message.

Keep the message short and sweet. Do not send 5 paragraphs about your business model. I’ve been on the receiving end of the 5 paragraphs of your business model, as have so many of the ISPs that it’s turned into a joke among delivery: “Let me tell you about my business model…” They don’t care, they just want to know what the problem.
The message should have 3 (short!) sections.

1. State the problem: “Mail I am sending from IP address is consistently going to the bulk folder. These are [sales receipts / tickets / bills / newsletters].”
2. State what you’ve done to fix it:”We have changed our delivery in X fashion” (limited connections, improved data hygiene, stopped mailing very old addresses, fired the idiot sales guy who decided spamming was a good idea, whatever it is).
3. Ask for a resolution: We’d like to know what you are seeing from our mail server that’s causing you to think this mail is unwanted by your recipients. I’ve attached a copy of the blocked / bulked message.

You MUST include the sending IP address in all correspondence. I can’t emphasize this enough. Without the IP, no one can help you. Without the IP they may even not bother to answer you. Without the IP the only response you will get it “what’s the IP?”
Also, don’t try and call. I know a lot of people prefer using phone to email, but in this case, use email. Calls are mostly useless.
The biggest issue is that getting an IP address over the phone is horrible. But when the IP is in an email, it’s a simple cut and paste into the internal tools. But there are also communication and documentation issues. Some ISPs like to have records of discussions about blocking and unblocking. On the communication level, when things are written down then no one is relying on faulty memories or hastily written notes about what needs to happen.
At smaller ISPs or even some small businesses, you can ask your recipients to talk to their support desk or admin.If the ISPs have customers telling them the email is wanted they’re much more likely to make filtering adjustments.

Blocking • postmaster • unblocking