I just got bitten by a similar issue. The scrollbar only appears when hovering the cursor in the window, and it is really easy to overlook.
If the scrollbars are not automatically showing, then see @skybat post in the following Thread.
Note:- I use Textmate on Mac or Notepad on Windows to manually edit any Themes.
If the Attachments scrollbar is still hard to see using any Theme, then try using the “wider scrollbars” in Settings so it stands out more.
If you have V10 go to “Menu / Settings (Preferences) / Appearance / Lists” and select “Use wide scrollbars” and then click “Apply” and Save and Close Settings.
If you have eM Client V8 or V9, you can adjust the scrollbar width via “Menu / Tools / Theme Editor”.
First go to “Menu / Settings (Preferences) / Appearance / Themes” and at the bottom click “Advanced / Save Current Theme As", and save your theme in your eg: “Documents / eM Client” folder.
Then open the “Theme Editor” in eM Client via the Menu and “Open your saved current Theme”. Next click “Scrollbar” on the left, and check the box “UseSystemScrollbar” & check the box as per the screenshot below. Then “save the theme” in your eg: “Documents / eM Client” folder.
Then in eM Client go to “Menu / Settings (Preferences) / Appearance / Themes”, and at the bottom click “Import Theme” and select your saved theme from your “Documents / eM Client” folder.
You will then see your custom theme appear “at the bottom of the builtin Themes”. Finally “click on your custom theme” and click “Apply” & Save & Close to exit Settings.
Lastly there is also alot of really good “User created Themes” you might like to try which also show the scrollbars really well.
(eM Client user created Themes)