If you only want the SMTP address (which appears in your example to always be tranferred using the X400 protocol) then something like the following may form a suitable basis for work:

$line = "SMTP:makeusbetter@domain.com%X400:c=US;a= ;p=domain ;o=domain
+;s=makeusbetter;%CCMAIL:makeusbetter at domain%";

if ($line =~ m/SMTP\:(.*)\%X400/) {
    print "Email address is: $1\n";
}
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Arun