McAfee blocks SMTP connections that do not operate on the default port 25. If you have Outlook, Outlook Express, Windows Mail or Windows Live Mail configured to use an SMTP port that is not 25 then you get errors such as:
Your server has unexpectedly terminated the connection. Possible causes for this include server problems, network problems, or a long period of inactivity. Account: ‘account_name‘, Server: ‘server_name‘, Protocol: SMTP, Port: 587, Secure(SSL): Yes, Error Number: 0x800CCC0F
I understand that this error also occurs in other flavours of McAfee antivirus software, not just the Total Protection variety.
To fix you need to stop McAfee scanning your email before it arrives at the inbox. In McAfee Total Protection for Small Business (TOPS) this is done on the web portal and by visiting Groups + Policies > Policies > Edit Policy for the policy that your client with the problem is using > Advanced Settings and untick Scan email (before delivering to Outlook Inbox).
You need to wait a while (hours) before this setting will take effect, so try again tomorrow and see if the problem has gone away having unticked the option today!