To block sender email address or domain name Please login to cpanel and do the folowing steps.
Mail –> SpamAssasin –> Configure SpamAssassin
To block a single email address (for instance example@domain.com)
Enter example@domain.com in to the “blacklist_from” field.
To block an entire domain (for instance domain.com)
Enter *@domain.com in to the “blacklist_from” field
To whitelist email address or domain name:-
Mail –> SpamAssasin –> Configure SpamAssassin
To block a single email address (for instance example@domain.com)
Enter example@domain in to the “whitelist_from” field.
To block an entire domain (for instance domain.com)
Enter *@domain.com in to the “whitelist_from” field
Tricks For Linux 
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