The machin still seems to be infected, based on the mass mailing block events. What's the next step if McAfee doesn't pick anything up?
But McAfee _is_ picking something up. You posted the Event log entry indicating Access Protection just stomped on an email attempt.
You need to review the Access Protection log file on that client. Get the details of what's being blocked, specifically the Process name.
That process is the lead you need for finding the alleged malware.
Worth noting is the mail attempt was made using SYSTEM credentials; so ensure you have administrative credentials when trying to investigate further.
Try to scan the client with Malwarebytes, it usualy picks up brand new viruses way faster than Mcafee. Otherwise use also wwarrens tip for getting to find the mail sending component of the virus.