Listed on SORBS DUHL
Your IP is on the SORBS Dynamic User and Host List (DUHL), which lists residential and dynamic IPs that shouldn't be sending direct mail.
A DUHL listing suggests your sending IP looks like a consumer connection, not a mail server. Many corporate receivers reject or heavily filter DUHL-listed sources. Open and reply rates degrade quickly.
What causes it
- You're sending from a residential or dynamic IP range (home broadband, VPS without rDNS).
- Your IP's reverse DNS looks residential (e.g., c-71-42-x-x.hsd1.ca.comcast.net).
- You're on a cloud provider's generic IP pool with no rDNS set.
How to fix it
Step 1: Set proper rDNS (PTR) on your sending IP
Your IP should resolve to a mail-server-looking hostname (mail.yourdomain.com), and that hostname should A-resolve back to the same IP. If you're on a VPS, ask the provider to set the PTR record for you.
Step 2: Request removal
Visit sorbs.net and request removal from the DUHL. SORBS will verify your rDNS is set correctly before removing the listing.
Step 3: Reconsider self-hosting
If you're sending more than a few messages a day, DUHL listings are a sign you should use a dedicated transactional email service instead of self-hosted SMTP. The deliverability engineering is their job, not yours.
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