Go to your back office > Dashboard > Settings > Order email settings and click on the checkbox in front of "Please confirm that you have added Diese E-Mail-Adresse ist vor Spambots geschützt! Zur Anzeige muss JavaScript eingeschaltet sein! and Diese E-Mail-Adresse ist vor Spambots geschützt! Zur Anzeige muss JavaScript eingeschaltet sein! 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.