If you have several email addresses linked to eM Client, the first one that you have added will be automatically set as default. Default account will be selected as sender when you compose messages in a neutral place, such as the Favorites or Local Folders, for example. You can change which email account you’re sending messages from just by clicking on your email address in the New Message window.

If you want to change your default account so you don’t have to perform this step every time, go to Menu > Accounts, select the preferred account from the list on the left side and then click the ‘Set as default’ button. The account marked by a star is the one set as default.

Read more in the Documentation: Accounts