It is! I'm happy to help clarify how it works. In SmarterMail 16.x, we've added two features that users have been requesting. For aliases, we've added "Allow users to send from this alias." When enabled, users can manually change their From address to the alias address by using the Send From dropdown on new emails and replies. (This is a manual change to prevent confusion for users who do not wish to reply from their alias address.)
In addition, when an alias allows users to send from its address, the Domain Administrator can go to the Domain Settings > Signatures section and choose to require a specific signature with that alias or allow users to map their own.
We've also added an option in a user's Account Settings for "Use To: address for replies." This feature is geared toward users who wish to reply automatically from their alias address; however, it also applies to some other cases as well. When enabled, any reply that you send via webmail will use that email's To: address as the From address. This setting applies to email aliases, domain aliases, plus addresses, disposable addresses and SMTP accounts. For example, when this setting is enabled and you reply to a message sent to an alias you're part of, the alias address will automatically be used as the Send From address. (You can manually change it back if you wish.)
Let me know if you have any questions or problems! From our local testing, things have been working well with these additions.