This is probably linked to this know issue on 5.7.1: https://jira.nuxeo.com/browse/NXP-11990

The easiest workaround is to configure a mail server on your Nuxeo instance.

Otherwise, you can add the following content to a file named social-collaboration-chains-config.xml in your $NUXEO_HOME/nxserver/config folder to override the default chains contribution of the Nuxeo Social Collaboration module with the fixed ones:

<?xml version="1.0"?>
<component name="org.nuxeo.ecm.social.workspace.operation.chains.override">

  <require>org.nuxeo.ecm.social.workspace.operation.chains</require>

  <extension target="org.nuxeo.ecm.core.operation.OperationServiceComponent" point="chains">
    <chain id="SocialWorkspaceCreatedChain">
      <operation id="Context.FetchDocument"/>
      <operation id="Context.SetVar">
        <param type="string" name="name">admins</param>
        <param type="object" name="value">administrators</param>
      </operation>
      <operation id="Workflow.CreateTask">
        <param type="string" name="task name">validateSocialWorkspace</param>
        <param type="string" name="directive">VERIFICATION</param>
        <param type="string" name="comment">Please validate social workspace</param>
        <param type="string" name="variable name for actors prefixed ids">admins</param>
        <param type="boolean" name="create one task per actor">true</param>
      </operation>
      <operation id="Notification.SendMail">
        <param type="boolean" name="rollbackOnError">false</param>
        <param type="string" name="from">expr:Env["mail.from"]</param>
        <param type="string" name="message">&lt;html&gt;
Hello,

Social Workspace &lt;a href="${docUrl}"&gt;${Document.title}&lt;/a&gt; was created. Please validate it &lt;a href="${baseUrl}nxadmin/${Session.repositoryName}${Document.path}@view_admin?tabIds=MAIN_TABS%3Aadmin%2CNUXEO_ADMIN%3ASocialWorkspacesAdministration"&gt;here&lt;/a&gt;.
&lt;/html&gt;</param>
        <param type="string" name="subject">Social Workspace  created</param>
        <param type="string" name="HTML">true</param>
        <param type="stringlist" name="to">expr:Fn.getEmailsFromGroup("administrators")</param>
      </operation>
    </chain>
    <chain id="SocialWorkspaceNotValidatedChain">
      <operation id="Context.FetchDocument"/>
      <operation id="Document.SetLifeCycle">
        <param type="string" name="value">delete</param>
      </operation>
      <operation id="Notification.SendMail">
        <param type="boolean" name="rollbackOnError">false</param>
        <param type="string" name="from">expr:Env["mail.from"]</param>
        <param type="string" name="message">Social Workspace "${Document.title}" was not validated by adminitrators and it will be suspended.</param>
        <param type="string" name="subject">expr:Social Workspace not validated: @{Document.title}</param>
        <param type="stringlist" name="to">expr:Fn.getEmail(Document["dc:creator"])</param>
      </operation>
    </chain>
  </extension>

</component>