Scenario
When a user removes a campaign domain or marks it as Inactive, the images and links are broken when the campaign is sent out. This also happens if the campaign domain is not selected on the campaign template.
Solution
The problem is caused by the domain created which was saved as a Default Campaign Domain and when there is no email campaign domain from Domains list, the default one is used, which is incorrect.
To resolve:
1. Navigate to Setup > Marketing > Marketing Preferences
2. Click General
3. Default Campaign Domain: Select an empty value
4. Click Save