When Inky sets up an account it automatically has a digital signature associated with the email. In order for Inky to digitally sign or encrypt a message, it needs to have the password of that email account. Since there are no password associated with an alias email- Inky cannot sign or encrypt a message from an email alias. If you wish to send an email as an alias- you need to turn off digital signature and encryption.
Inky does not automatically show a digital signature button for a setting in the compose window, but you can change the settings to add it. Here's how:
From the Inky home screen: go to Settings-->General--> then in the right pane under "Composer" slide the toggle for "Show the option to control the digital signing of individual drafts" to the right. When you exit the settings and go back to compose an email- there will be a new button called "Sign" beside "Encrypt". The "Sign" button will be automatically activated, but you can click the check mark "off" and at that point be able to send mail as an alias.