Hardens the app against a message built to hurt it: a calendar invitation could freeze the reader for good, and a message split into a flood of parts could run it out of memory. Attachments and inline images now have a size ceiling, and a message too large to cache is refetched instead of becoming permanently unreadable. On the security side, an address carrying a line break is refused before it can reach the SMTP envelope, importing K-9 settings that do not state a connection security now assumes an encrypted one instead of cleartext, choosing "None" says what it costs, a shared file is only accepted through a content URI, and server responses are no longer quoted into logs. Also: adding an identity no longer changes who you send as, new mail announces itself once instead of twice, Enter turns a recipient into a chip and keeps the cursor in the field, Snooze is gone from Spam and the Trash, and Save stays dim until something has actually changed.
