Why are the emails from Apple Mail in this numbered directory structure?
It looks like an Universally Unique Identifier (UUID) is a 128-bit number used to identify information in computer systems.

Anyone can create a UUID and use it to identify something with near certainty that the identifier does not duplicate one that has already been created to identify something else, and will not be duplicated in the future. Information labeled with UUIDs by independent parties can therefore be later combined into a single database, or transmitted on the same channel, without needing to resolve conflicts between identifiers.

In its canonical string representation, the sixteen octets of a UUID are represented as 32 lowercase hexadecimal (base 16) digits, displayed in five groups separated by hyphens, in the form 8-4-4-4-12 for a total of 36 characters (32 alphanumeric characters and four hyphens). For example:

123e4567-e89b-12d3-a456-426655440000