> [...]

Further to the responses you have received already, I'd like to note that if you
want to receive mail at {alias}@{user}.mydomain.com then, at the time of
*sending* the e-mail there needs to be an MX record for user, as otherwise the
sender won't be able to connect to your (postfix) server.

That means that the users will have to exist *before* postfix receives the
message, and thus clearly before dovecot receives it.. so you may have to
reconsider your requirement of adding users of on-the-fly.