How to reproduce:
Compose new email or answer email, try to add recipient from address book
Several of my subjects had big difficulties to add a recipient whose email address was in the address book.
This means that they searched very long for the function/button and tried out various buttons (see screenshots):
First thing they did was to try adding the recipient by clicking on the „+“ button next to the recipient line - but this only offers the options CC, BCC etc. They then tried to click on „contacts“ in the navigation column on the left but this would mean leaving the currently open email and thus they had to break this off. They then tried to use the „To+“ etc. buttons in the lower left corner.
Both of the subjects I refer to finally added the recipient by double-clicking on corresponding contact in the „contacts“ button in the column on the left (not the navigation column, but the second column from the left) - but it took them quite some time to find this button.
Apart from auto-complete there should be an intuitive way to add a recipient from an address book. Since participants first of all tried to use the „+“ button for this, it might the best thing to add another „+“ button next to the existing one (like we also have a drop-down-arrow and a pen to edit the sender in the line above): Ideally, the different functions of the two + buttons will be understood immediately; for this purpose you could make the second + button a + with the head of a person next to it, like the buttons „To+“ etc (if tried to make clear what I mean in the screenshot)., which makes clear that this refers to adding contacts!