Steps to reproduce:

* CASE A *
1) Select the first 4 email
2) Press "DEL"
-> the selected emails are deleted
-> the first email on the list is selected
3) SHIFT+Click on the second email on the list
-> the first five email are selected

* CASE B *
1) repeat steps A1 and A2
2) SHIFT+Click on the fifth email of the list
-> The first and the fifth email of the list are selected, the second, the third and the fourth aren't.

* CASE C * 
1) repeat steps A1 and A2
2) SHIFT+Click on the fourth email of the list
-> The first, the fourth and the fifth email of the list are selected, the second and the third aren't.

Recap:

1. Click on email 1

2. SHIFT+Click email 4 
[1..4] are selected

3. Press DEL
[1] is selected

4/A. SHIFT+Click email 2
[1..5] are selected instead of [1,2]

4/B. SHIFT+Click email 5
[1,5] are selected instead of [1..5]

4/C. SHIFT+Click email 4
[1,4,5] are selected instead of [1..4]


There are even other cases, for example starting the selection not from the first item of the list, or from the last...

Thanks for the attention.