When you email a document (i.e. a sales order confirmation or purchase order) from Business Central, the email address from the contact person of that document will be selected in the 'To' field.
If you then want to add another email address, you need to manually enter the entire address (or copy & paste it from somewhere).

It would be nice if you can select emailadresses you have saved in Outlook, or that addresses you regularly use would be auto-completed after typing the first letters (just like in Outlook).