Fastmail lets you create wildcard identities like this so you can send from any username at any domain you have with them, but if you are sending from a third party app you usually still need to set up the sending identity in the app itself, which is annoying. The email programs I've tried haven't let me type arbitrary addresses into the 'from' line.
Many programs won't even automatically reply from the same alias the message was received at.
Many programs won't even automatically reply from the same alias the message was received at.