It depends on the alias service you use (this appears to just give you another frontend to your alias service, eg, Addy.io or Firefox Relay). I know with Addy.io, forwarded emails have a special "Reply-To" header which is an address that Addy.io monitors and will forward your response back to the original sender. So replying to email delivered to your alias isn't a problem, though I think initiating an email from an alias would be tricky.
It’s possible to initiate from Addy as well. For example, if your Addy alias is example@mailer.me and you wish to email hn@gmail.com then you would send an email to example+hn=gmail.com@mailer.me from the email address registered to the Addy account.
