I'm not sure where to report Outlook.com's IMAP bugs, so if anyone knows a better place, please let me know.

ENVELOPE retrieved for a message, that has national characters in subject (correctly encoded using Base64), has '?' characters instead of encoded characters.

Actual:
ENVELOPE ("Fri, 27 Sep 2013 17:59:48 +0200" "za????"

Expected:
(hmailserver, aol, dovecot):
ENVELOPE ("Fri, 27 Sep 2013 18:23:23 +0200" "=?utf-8?B?emHFvMOzxYLEhw==?=" ...
(gmail):
ENVELOPE ("Fri, 27 Sep 2013 18:21:17 +0200" "=?ISO-8859-2?B?emG/87Pm?=" ...


Here are the logs:

Mail.dll C: 6d9d6c00b9294efb APPEND Inbox (\SEEN) {243}
Mail.dll S: + Ready
Mail.dll C: Content-Type: text/plain;
 charset="utf-8"
Content-Transfer-Encoding: 7bit
MIME-Version: 1.0
Subject: =?utf-8?B?emHFvMOzxYLEhw==?=
Message-ID: <*** Email address is removed for privacy ***>
Date: Fri, 27 Sep 2013 17:59:48 +0200

Hello
Mail.dll S: 6d9d6c00b9294efb OK APPEND completed
Mail.dll C: fdfd55add5774f3c SELECT "INBOX"
Mail.dll S: * FLAGS (\Answered \Flagged \Deleted \Seen \Draft)
Mail.dll S: * 3 EXISTS
Mail.dll S: * 0 RECENT
Mail.dll S: * OK [UIDVALIDITY 368082] UIDs valid
Mail.dll S: * OK [UIDNEXT 100066] Predicted next UID
Mail.dll S: * OK [PERMANENTFLAGS (\Answered \Flagged \Deleted \Seen \Draft)] Limited
Mail.dll S: fdfd55add5774f3c OK [READ-WRITE] SELECT completed.
Mail.dll C: 170583054a6a4cf3 UID SEARCH ALL
Mail.dll S: * SEARCH 100065
Mail.dll S: 170583054a6a4cf3 OK SEARCH Completed
Mail.dll C: 0a55ca5702664b94 UID FETCH 100065 (UID RFC822.SIZE INTERNALDATE FLAGS ENVELOPE BODYSTRUCTURE)
Mail.dll S: * 3 FETCH (UID 100065 RFC822.SIZE 290 FLAGS (\Seen) INTERNALDATE "27-Sep-2013 15:59:44 +0000" ENVELOPE ("Fri, 27 Sep 2013 17:59:48 +0200" "za????" NIL NIL NIL NIL NIL NIL NIL "<*** Email address is removed for privacy ***>") BODYSTRUCTURE ("TEXT" "plain" ("charset" "utf-8") NIL NIL "7BIT" 5 1 NIL NIL NIL NIL))
Mail.dll S: 0a55ca5702664b94 OK FETCH Completed

-- 
Pawel Lesnikowski