jelmilter uses milter4j to implement several mail filters:
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HeloCheck: A MailFilter that verifies the connection endpoint (mail server / MTA) against the submitted [E]HLO parameters.
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RegexCheck: A MailFilter that checks emails against regular expressions or for the presence of specific strings.
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WhoisCheck: A MailFilter that extracts URLs from an email and checks the corresponding WHOIS records of the related site using a whois-spam server. Since more and more domain owners and registries now hide ownership information, this filter is no longer very useful in practice. However, it still serves as a good example of how to implement custom MailFilters.