Symptoms:SharePoint Designer 2007 created workflow will "stop" before the Send Email action. This may only happen for certain users, therefore, making the issue seem to happen intermittently.Workflow status error: "Stopped"
Send Email action has SharePoint group(s) and/or workflow lookups in the To / Cc line.
Make sure each SharePoint group has at least Read permissions for the site. Additionally, that in the Group Settings area of the Group Page, under Who can view the membership of the group, Everyone is selected.
Office Online - Send e-mail in a workflow