If you want your alias email address to be the default address in eM Client for sending mail, then "set the alias email address (to be the email address) in the “Accounts / General Tab” under your Name as @Gary advised in his post in this thread. Then save accounts.
As @makani advised, some mail servers also require you to change the Alias email address “on the server end to be the default” otherwise it will just default back again when the mail server end sends it. So check in your online mail server settings. If you don’t know where that settings is located, ask your mail server tech support.
For eg: Gmail users, apart from changing the alias email address in eM Client accounts General tab, "you also need to “change that in Gmail online”.
You need to change both your default “From” and “Reply-to” address in Gmail online.
If you only change the “From” address, replies will go to your original Gmail address by default.