Luke Le on 07 Jul, 2018 01:59 PM
Hi B A,
this sound peculiar. The only reason I could think of why that would have changed anything is, that macOS had an older cached version of the defaults which didn't have the proper DefaultSecurityMethod set. And that by deleting a different key from the defaults macOS evicted the cached version and loaded the new one.
What is definitely necessary is restarting Mail.app after changing the defaults.