Please follow below steps to change the default dictionary in your Outlook.

1. Click File > Options to open the Outlook Options dialog box.

2. In the Outlook Options dialog box, please click Mail in the left bar, and then click the Spelling and Autocorrect button in the Compose messages section. See screenshot:

3. Now the Editor Options dialog box comes, please click the Custom Dictionaries button. See screenshot:

4. In the opening Custom Dictionaries dialog box, please select the new dictionary you will set as default in the Dictionary List box, and click the Change Default button. See screenshot:

5. Click the OK buttons successively to close all dialog boxes.

So far, you have change the default dictionary in your Outlook already.

Automatically reply with the default email account in Outlook

In general, Outlook can identify the email account that the active email belongs to, and then reply with this email account automatically. However, with Kutools for Outlook's Always reply with default account option, no matter which email account the active email belongs to, it will be replied with the default email account in you Outlook.

Add and remove words in custom dictionary in Outlook