Seen in Outlook 2013:
If you write a new mail, enter a valid eMail adress, mark the address and start writing a new eMail adress, a bug notification pops up directly.
The same happens when answering a mail, marking the address, start writing some text, and then breaking off, i.e. pushing Esc twice.
The same happens when selecting a sent mail, "edit as new", marking the address, start writing some text, and then breaking off. (Likely by the same mechanism as in the 2nd case, but different to the 1st case)