List top sending sources (ESPs, ISPs, mail services) for a domain, grouped by source type.
Filters: "known" (legitimate ESPs like Google, Mailgun), "unknown" (unrecognized senders), "forward" (forwarding services). Empty = all types.
Returns top 20 per type with message volume, SPF/DKIM/DMARC pass/fail counts.
Use this to investigate WHERE email is being sent from — especially when unknown sources appear or compliance is low.
To drill down into a specific source (by IP, ISP, hostname, or reporter), use get_domain_source_details.
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