In my case the mail server responded with the following after selecting
a mailbox (in the Mail application):
* OK [CLOSED] Previous mailbox closed.
* FLAGS (\Answered \Flagged ...)
* OK [PERMANENTFLAGS (\Answered \Flagged ... \*)] Flags permitted.
* 2 EXISTS
* 0 RECENT
* OK [UIDVALIDITY 1234567890] UIDs valid
* OK [UIDNEXT 12345] Predicted next UID
* OK [HIGHESTMODSEQ 123456] Highest
A6 OK [READ-WRITE] Select completed (0.002 secs).
The [CLOSED] part threw the parser off as it was expecting a space after
the atom following the opening bracket, which would actually lead to a
crash of Mail (AK::Optional::value() without value).
This commit doesn't include support for FETCH BODY, because it's a bit
big already. Rest assured, FETCH is the most complicated IMAP command,
and we'll go back to simple boring ones shortly.