Assume that there's an email that has more than 12 recipients in a Microsoft Exchange Server 2013 environment. In this message, the email header contains an email address that contains the "from" attribute and an email address that contains the "reply-to" attribute of the sender's actual email address. When you select the Reply button in Microsoft Outlook Web App, the "from" address replaces the "reply-to" address to be displayed as the sender's address in the From field of the reply email.
This issue occurs because the mail compose component switches subsequent function calls when an email has many recipients. The subsequent function calls cause the Reply-To property to be omitted. Therefore, the "from" email address is populated as the sender's address.
To fix this issue, install Cumulative Update 10 for Exchange Server 2013.
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.