In /etc/amavisd.conf, Section V covers white/blacklisting of sending email addresses. It can be done with a list of email addresses in the file as an ACL (an example being given in the commented line that begins: #@whitelist_sender_acl =), a regular expression or series of regular expressions, or a reference to an external file (see the example line beginning #read_hash(\%whitelist_sender) which is then converted to a hash for address testing.
Starting out, just creating an ACL list in the body of the amavisd.conf file should be enough. All you have to do is change:
# @whitelist_sender_acl = qw( .example.com );
to (use the sender’s real domain name rather than sendersdomain.com)
@whitelist_sender_acl = qw(sendersdomain.com);
And restart at least amavisd, although it is probably easier to do a:
serveradmin stop mail && serveradmin start mail
This should exempt any email from the sendersdomain.com domain from the SPAM checks.