How do I initialise the async engine?

Call md_async_init once at package scope (outside any sub), after the use statements at the top of your filter file. It sets up the AnyEvent event loop that all async checks share. Requires the optional CPAN modules AnyEvent, AnyEvent::DNS, AnyEvent::Socket, and AnyEvent::Util.

use Mail::MIMEDefang::Async;
use Mail::MIMEDefang::Async::Checks qw(
    md_async_check_dnsbl
    md_async_check_rdns
    md_async_check_spf_record
    md_async_check_dmarc_record
    md_async_check_mx_exists
);
use Mail::MIMEDefang::Async::Results qw(
    md_async_interpret_dnsbl
    md_async_interpret_rdns
    md_async_interpret_spf_txt
    md_async_interpret_dmarc
    md_async_score_results
);

md_async_init(
    max_concurrency => 8,   # max parallel checks in flight
    global_timeout  => 10,  # hard wall-clock limit for the whole batch (seconds)
    dns_timeout     => 5,   # per-DNS-query timeout (seconds)
    socket_timeout  => 5,   # per-socket-connection timeout (seconds)
);

These settings apply to every call to md_async_run_checks() in the filter. Tune global_timeout to stay within your MTA's milter socket timeout.