Consider the following scenario:
The Microsoft Lync instant messaging (IM) feature is enabled for Outlook Web App in a Microsoft Exchange Server 2013 environment.
You create an email message that contains more than seven individual recipients (excluding distribution lists and external recipients). Then, you send the email message.
A recipient opens the email message in Outlook Web App, and the recipient list in the email message is collapsed. Then, the recipient opens a chat window by clicking the Reply all by IM button.
In this scenario, only seven recipients appear in the chat window.
This issue occurs because the Reply all by IM button only checks one specific collection that contains up to seven recipients in the list.
To resolve this issue, install the following service pack:
2926248 Description of Exchange Server 2013 Service Pack 1
To work around this issue, you can expand the collapsed recipients list before you open the Reply all by IM chat window. Then, all recipients are added to the group chat.
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.