Setup Alias on K9

First of, open K9 and go to your account settings

When in Settings, go to "Sending Mail" tab, tap on "Manage Identities".

Select "New identity" by tapping the "three dot" icon on top right.

(Every Disroot user has an username@disr.it alias to use by default)

And fill in the form providing the new alias address.

Set default

To change the default identity, while still in "Manage Identities" settings, just tap and hold the alias you want to set and select "Move to top / make default" option.

Send email

To send email with your new alias, just tap on the "From" field and select alias you want to use from the dropdown menu, when composing your mail.