Go to your back office > Dashboard > Settings > Order email settings and click on the checkbox in front of "Please confirm that you have added Dit e-mailadres wordt beveiligd tegen spambots. JavaScript dient ingeschakeld te zijn om het te bekijken. and Dit e-mailadres wordt beveiligd tegen spambots. JavaScript dient ingeschakeld te zijn om het te bekijken. to your email's White list to ensure email delivery."
Make sure you add those email addresses to your Inbox's White list.

Set "Enable Custom Invoice number: Yes"
Use these options to construct your order numbering:
Y - the current year in 4 digits
y - the current year in 2 digits
m - the current month in 2 digits
d - the current day in 2 digits
n - the subsequent number of the invoice for each day
Please use "n" as a mandatory option to avoid Order number duplicates which will create problems.

Make sure you "Save" the settings after you are done editing.