What users knew how to do before using Outlook, they forget after using it.
Email used to be a simple affair. You type in an address, subject and message body content, and hit send.
Now, with Outlook, everything seems to be complicated. It tries to help by looking up the first recipient it has seen to send the message to (often with disastrous results). It makes you think it is looking up recipients in the address book that is built in when it auto-finds recipients (when in fact it is not).
It makes users think that they need to create a 100 MiB document as an attachment instead of using the crappy default of using a MS Word canvas without most of the features of the MS Word editor, and when using the MS Word editor default, it usually fails by stuffing it in some retarded rtf embedded in a wrapper turd attachment.