chore: re-license under MPL-2.0
doc: add doc comments and examples
fix: assert wait for all clients to finish
refactor: assert return bool instead of using assert!
fix: allow empty bytes in body_contains
refactor: internalize Mailbox
fix: hide Mailbox.authenticate from public
test: set timeout to fail faster
feat: allow clients to reconnect after QUIT
refactor: allow multiple recipients in tests
refactor: rename Mail.recipient to recipients
refactor: use HashSet for recipients
refactor: default test builder domain
refactor: group mail transaction fields
refactor: use Option<String> for user/password
refactor: no verify creds in tests with default auth user
feat: keep track of mail body