Why do traffic reports not display sent emails for some users?

Question:

I ran a report of traffic on a selection of mailboxes for a 6-month period, but some of these mailboxes appear to receive messages and they do not send any.
I know this is not true. How come?

Answer:

Please make sure that these mailboxes do not send email using a non-MAPI email client such as Outlook Express, Eudora or Mozilla ThunderBird, see the following article: Traffic of users of non-MAPI email clients are missing in my reports.

Note: Non-Outlook (non-MAPI) clients are SMTP clients. OWA is not included in that category (OWA traffic being also MAPI traffic).