Choose subject for Outlook mail

Is it possible to make a dropdown menu in outlook mail for selecting subject title from it? Subject titles will be taken from an excel list. Near the subject title that has come automatically into the subject line, I would like to be able to type something else.

Thanks in advance,

No, it is not. The Outlook extensibility model doesn't provide anything for that.

As for the Ribbon, Outlook doesn't support Fluent UI customizations from VBA macros. You may consider developing an add-in instead where you can:

1. Add a custom ribbon controls. See Walkthrough: Creating a Custom Tab by Using the Ribbon Designer.
2. Create a form region where you can choose subject lines. See Creating Outlook Form Regions.
3. Use Windows API functions for subclassing Outlook windows and placing custom forms on top of the window (right below the subject line). See Creating Adjacent Windows In Outlook .
4. Or use third-party components for creating such forms. See Advanced Outlook view and form regions for more inforamtion.