The original time stamp on a sent message is the time on the sending client - her PC.
Each point along the way adds a time stamp as well, and again it is that machine's time setting that is used (Exchange server - SMTP server - recipient client). Right click a message in her Inbox and select Options to see the message header with time stamps.
Next, the sending client may be configured as to when a message is sent - immediately, or at the next scheduled Send/Receive event. This is found in Outlook/Tools/Options/Mail Delivery.
The same applies to the Exchange server and every external SMTP server in the path to the recipient. Messages may sit on a server until a scheduled time interval.
Meeting reminders as email follow the above rules. Local reminders based on Calendar entries are set per entry. The default is set in Outlook/Tools/Options/Preferences/Calendar Options. Again, the client clock is the time source.
Outlook can get flaky as the folder size increases. Archive on a regular basis and keep the number of messages in the Inbox to a reasonable number.