The EagleFiler Spotlight mail importer does not populate Spotlight fields like kMDItemSubject and other mail specific fields.
The default /System/Library/Spotlight/Mail.mdimporter does this for .emlx files.
MailMate’s spotlight importer does for .eml files.
It would be good if EagleFiler’s importer fully understood .eml files.
The Spotlight fields of interest include: kMDItemSubject, kMDItemPrimaryRecipientEmailAddresses, kMDItemAuthors, kMDItemAuthorEmailAddresses.
As a workaround, I have deleted EagleFiler’s importer and copied MailMate’s to /Library/Spotlight.