A clean domain is a data domain that is free of errors. Leads, accounts, contacts, and opportunity records have the domain name of that particular company. Domain name is nothing but the website name of the company. Each website is stored on a server with an Internet Protocol (IP) address, such as 69.63.191.255. Instead of seeing those IP addresses, we use easy-to-remember names in their place and that name is called the domain name.
The unique element of a URL is the domain. This is the critical component to pull out of any given URL that represents a unique entity, because there is only one "salesforce.com", "fullcast.io" or "google.com".

Figure 1. Clean Domain
Why is a clean domain important?
While updating details of the lead, account, or opportunity, sales representatives usually copy-paste the URLs. This kind of manual data entry is prone to duplicates. Also, there is the possibility that identical accounts have one or more different URLs but the same domain name like the below example:
example@gmail.com
If there was an entry for three different accounts with the same domains, there is no simple way to identify the duplicates. So it is essential to identify the Clean domain to execute BMA and account policy. To identify this clean domain we pass the records through the domain checker which pulls out the root domain of a given record from its repository of domain names.
Exception list in clean domain
In most companies, email addresses are structured with mailbox names and domains as the key elements. For example, every employee at Salesforce uses “salesforce.com” for their email address. So it is easy to pick the domain name from this. But some companies like Google, use both gmail.com and google.com as their email extensions. In those cases, the domain checker remains a failure so there is a need to provide an exception list. So those records that are provided in the exception list are not considered for domain check. The domain names for these records are taken as is.
Information:
Please click on the link to know the exception list for the clean domain. Clean-domain-list.xlsx