The selector to use for DKIM key identification.
For example, if 'selector1' is used here, then for each domain 'example.org' given in domain, 'selector1._domainkey.example.org' should contain the TXT record indicating the public key is the one in /var/lib/dkimproxy-out/public.key: "v=DKIM1; t=s; p=[THE PUBLIC KEY]".