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Use GnuPG from Homebrew
MailMate searches for the GnuPG binary using the default path from MacGPG. If you have GnuPG installed using Homebrew or any other method, you need to update the path through hidden settings, e.g.:
defaults write com.freron.MailMate environmentVariables -array '{ enabled = :true; name = "MM_GPG"; value = "/opt/homebrew/bin/gpg"; }'
Source: farrokhi