Symptoms
Assume that you create a new meeting request and send it to a user's alternative SMTP address that is stored in your nickname cache. If the recipient proposes a new time for this meeting, the Accept Proposal button is greyed out, and the proposal cannot be accepted when you open it.
Resolution
To resolve this issue, install May 12, 2015, update for Outlook 2013 (KB3039799), and then follow these steps.
Important
Follow the steps in this section carefully. Serious problems might occur if you modify the registry incorrectly. Before you modify it, back up the registry for restoration in case problems occur.
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Exit all Microsoft Office applications.
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Start Registry Editor:
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In Windows 7, click Start, type regedit in the Search programs and files text box, and then click regedit.exe in the search results.
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In Windows 8, move your mouse to the upper-right corner, click Search, type regedit in the search text box, and then click regedit.exe in the search results.
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Locate and then select the following registry subkey:
HKEY_CURRENT_USER\Software\Microsoft\Office\15.0\Outlook
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On the Edit menu, point to New, and then click DWORD Value.
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Type AllowUnknownExAttendeesOutOfDate, and then press the Enter key.
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In the Details pane, right-click AllowUnknownExAttendeesOutOfDate, and then click Modify.
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In the Value data box, type 1, and then click OK.
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Exit Registry Editor.
