RBL - Realtime Black List

Learn about how we block spam, and how email may be blocked by other ISPs also.

Any email that is received by our servers may be blocked by our servers as spam. (eg inbound email from your customers and friends)

Any email that you send, may be blocked by the recipient ISP servers for the same reason. 

How the decision to block email is made, has many steps

  1. RBL: Is the sender of an email currenlty on a real time black list (RBL)
  2. Spam Filter: Does the email meet the threshold to be considered spam, by anti spam software such as "spamassasin" (eg links, keywords, asking for money etc) 
  3. Policy: Is the sender email or IP sending too many emails too quickly

Realtime Black List (RBL)

Most email providers uses real time black lists. There are hundreds of popular blacklists. 

We use the following RBL 

  • barracudacentral
  • spamcop
  • abusix   
We are no longer uses these (but may use in future)
  • spamhaus
  • abuseat

If you send an email and it is rejected, the likely reason is the RBL used by the recipients ISP. We do not filter outbound email by RBL, we trust our customers to use our SMTP services fairly. 

If your customers have an issue where their email is rejected by our mail servers, then you may need to let us know. The rejection email usually contains enough information for you to do a google search, and confirm that the IP is blocked, and why it is blocked. And the ability to request delisting. It is quite common for legitimate people to be blocked when sending, from time to time. It probably means that someone else who uses the same mail server as them was sending spam. The reputation of a mail server is only as good as all the users using it. 

If you want to consider if the IP address is ours, or someone elses, review our IP addresses

In order to provide support on RBL issues, we need all the information that is in the rejection email. We need to know who it was from, who it was to, the date/time, the IP address blocked, and usually there is a name of the RBL service that caused the block. 

In the following example, the senders blocked IP is "" and the RBL is "abuseat"

eg  554 5.7.1 Service unavailable; Client host [] blocked using cbl.abuseat.org

How block lists work under the hood, depends on the provider, but basically anyone can report an IP as abusive
Then the owner of the IP (if reputable) needs to manually request delisting, often with a time penalty that will be more or less depending on historical reputation. 
Some black lists just impose a short term block and auto delist within 24 hours. 
The problem with RBL, is there is alot of trust in this system. Essentially, hackers can start registering good IP's for exclusion, just to be annoying. 
However, if it was too hard to add an IP, then the system would not work, because it's the short term nature of the IP reputation that is being queried, because the period of spamming may only be for a few hours. Ordinary people need to be able to mark any email they receive as spam. But ordinary people can either accidently mark legitimate newsletters as spam, or unethical people can deliberately destroy the reputation of perfectly good servers. 

Spam Filtering

We use amavis and spamassassin to filter the email even more. These services look for keywords in the email, and determine a spam score, based on the following considerations

  • asking for money
  • typical spam keywords like pornography, pharmacutical products, SEO etc
  • links in emails are more likely considered spam
  • attachments are checked
  • attachments in emails without many personal comments are typically dangerous

The spam scores are present in your email headers, and if you have a good IT professional to help you, they can help you automatically move borderline spam into a spam folder on your inbox. We automatically delete emails that are definitely spam (with the highest spam scores)

Policy Filtering

We use a policy system to determine reasonable use.

Reasonable use might be sending no more than 200 emails sent per day from your email client. We can raise this limit, but you need to prove that you have good reason to send more emails. Please note the limit is not hard and fast, it is based on an algorithm that determines the speed that emails are being sent. Simply sending alot of emails in 1 hour could put you over the limit. However, most humans cannot possibly send 200 emails in a day.

We understand some customers may need limit increases for sending invoices from desktop software like MYOB.

If we raise the limit, then your email account exposes a reputational risk to our whole server. If your computer or mail account was hacked, and the hacker causes our email server to get a bad reputation, then this affects all our customers, and causes a financial risk to our business. The reason we tighten the limits is to ensure no individual customer mailbox poses a risk to the business. 

We also provide bulk email facilities within our CMS. This should be your preferred method of sending newsletters. 

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