- a/b/c will create mailbox c within b within a.
- a/b/d will create mailbox d in the same folder as c.
[robg adds: This is documented in Mail's help (click the question mark in the New Mailbox dialog), but not everyone may have read that. Also, I thought this might be a duplicate, but I couldn't find it in a quick search, so please let me know if I missed it somehow...]

