Distribute forms - check email inbox names

Hi, i have created a form in Acrobat X and need to distribute to a group of users. I have selected 'manually collect responses in my email inbox'. When i try to select the users , it reurns error (shown below). Though the users are listed, there is error and not included in the 'To' field.

Let me know the reason for this. Is this error only in Acrobat X pro. Do i need to check with latest version Acrobat XI  or DC.