I just started shifting my email archives from Mail to EagleFiler and discovered something odd. The first time that I import a mailbox with messages that have MailTags tags on them, EagleFiler adds a bunch of alphabetical tags (ex.: a, b, c, d, e) some of which are capitalized, others of which are not.
After I’ve deleted the extraneous tags (which are never attached to any messages that I can tell), subsequent imports of messages with MailTags tags on them don’t add the extra tags any more.