一位 Office 365 用户发现，大约有 100 封电子邮件似乎是由他发送的；这些邮件包含某种恶意 PDF 附件。邮件标题显示“已接收：来自 XXX.XXX.PROD.OUTLOOK.COM”作为电子邮件的初始来源。已发送的电子邮件会显示在 Exchange Online 邮件跟踪中，无论是内部收件人还是外部收件人。但是，它们不会出现在用户的“已发送邮件”文件夹中。
这是否证明有人成功入侵了他的账户（以他的身份登录）或者还有其他解释？
我需要了解是什么决定了一封电子邮件是否最终出现在已发送邮件中，以及一封发出的电子邮件是否可以出现在邮件追踪中，而无需有人以该用户身份登录。
我们已经更改了他的密码，并检查了他的电脑是否存在恶意软件。还有其他什么可以做的来防止再次发生吗？
更新：示例电子邮件标题仅略作修改：
Received: from MM1P123MB1050.GBRP123.PROD.OUTLOOK.COM (10.166.235.24) by
MMXP123MB1376.GBRP123.PROD.OUTLOOK.COM with HTTPS via
MMXP123CA0017.GBRP123.PROD.OUTLOOK.COM; Fri, 16 Mar 2018 09:33:43 +0000
Authentication-Results: [somedomain].co.uk; dkim=none (message not signed)
header.d=none;[somedomain].co.uk; dmarc=none action=none
header.from=[somedomain].co.uk;
Received: from MM1P123MB1034.GBRP123.PROD.OUTLOOK.COM (10.166.217.148) by
MM1P123MB1050.GBRP123.PROD.OUTLOOK.COM (10.166.217.152) with Microsoft SMTP
Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA384_P256) id
15.20.588.14; Fri, 16 Mar 2018 09:33:40 +0000
Received: from MM1P123MB1034.GBRP123.PROD.OUTLOOK.COM
([fe80::bd23:2882:93cc:c179]) by MM1P123MB1034.GBRP123.PROD.OUTLOOK.COM
([fe80::bd23:2882:93cc:c179%14]) with mapi id 15.20.0588.016; Fri, 16 Mar
2018 09:33:39 +0000
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From: somename lastname <somename.lastname@[somedomain].co.uk>
Subject: Important New Document
Thread-Topic: Important New Document
Thread-Index: AQHTvQgYiGQw1JKKkUqd6+Gw0vjPcg==
Date: Fri, 16 Mar 2018 09:33:39 +0000
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答案1
我想说是的，看起来有人拥有您的 Outlook.com 凭据，因为该电子邮件并未被欺骗，而是实际上源自微软。
两天前我们也遇到过类似的事情，一名用户在一封虚假电子邮件中收到一个虚假链接，链接指向据称位于 OneDrive 中的文档。点击该链接后，他们被带到一个虚假网站，要求输入凭证。输入凭证后，他们的 Outlook.com 帐户会被用来向任何可能被窃取的电子邮件地址发送电子邮件 - 如此循环往复……
为了帮助防止再次发生，2FA 和教育是您的选择 :-)