A blocklist is a list of IP addresses, domains or server that are suspected of sending spam. Usually recipient mail server use blocklists to help decide whether they should accept or reject an email. In such case, some email you send from your IP address may never be delivered if your IP address has been blocklisted.
The IP of your mail server will be blocked by the DNS blocklist monitor software if RBL report stated the IP is listed on the spam database. De-listing your IP may take days or even weeks. Anyone uses the software or firewall will not receive your email as your IP is considered as blocklisted.
**Note that RBL (Realtime Blackhole List) is a web site services that keep track of the domain name or IP addresses where possibly the spam emails originate from.
Why Domain is Blocklisted
1. Spam complaints from email recipients
The recipients may report the email as spam intentionally (they think the email is spam) or unintentionally (by clicking on “Spam” button instead of deleting the email). Some email recipients do not trust the unsubscribe option. They prefer reporting the email as spam in order not to receive emails from this sender anymore.
2. Send email to a spam trap email addresses
Some ISPs turn abandoned email addresses into spam trap mailboxes. Any message sent to a spam trap address is treated as spam. Most of the time spam trap email addresses are met in old and harvested email lists. If you look at a list that you have not emailed to for a year or two, it’s likely to contain many invalid and spam trap emails just like a list that you collect on the Internet.
3. Bad server and IP reputation
If you don’t have your own SMTP server and send emails from a shared mail server, your reputation depends on that server reputation. Sometimes ISPs blocklist the entire IP ranges, if an IP address from your IP block is suspected in spam activity, your IP may become blocklisted as well. As you see being put on a blocklisted is not necessary the result of a spam-like sending activity.
4. Site Apps is affected or Injected by Malware
Most virus are spread via email attachment, your IP may get affected once you click the link or attachment. In the same way, the sites that you visit may having malware hosted on their pages, it will send email spam from your domain or IP to the recipient. Therefore, you might get your domain into blocklisted list as security triggers detect malicious activity.
How to Check Blocklisting
Using multi-blocklist search tool called MXToolbox or any free online blocklist check tool to check whether your IP or domain is on the blocklist. If you find out your domain is listed, you may click on some suggestion or go to the individual database service and find out their policies and procedures for getting your domain removed from the blocklist.
How to Prevent Blocklisting
To keep your email out of the blocklisted list.
Use a static IP.
Set up an SPF record or DKIM.
Do not allow relaying.
Make sure you have a valid PTR record that matches your outbound public IP to your mail server name and hostname should resolve to the same IP address.
Make sure that your server is not on any RBL list.
Enable SSL for encrypting the connection between a mail client and the server.
Ask your subscribers to add your email address to their contact list.