Some websites block disposable email domains because temporary addresses can be used to create large numbers of short-lived accounts, which may increase abuse or spam. Blocking policies are usually based on domain lists and other signals. If a legitimate service rejects a temporary address, the appropriate solution is to use a permanent email address rather than trying to circumvent the website rules.
For practical use, always review the provider rules and avoid sharing sensitive information through temporary inboxes. A temporary address is best suited to low-risk, short-term interactions.