This patch groups a series of commands that redirect to a file into a
single grouped redirect, for readability reasons.
Suggested by shellcheck SC2129.
This patch implements "via" aliases, which let us explicitly select a
server to use for delivery.
This feature is useful in different scenarios, such as a secondary MX
server that forwards all incoming email to a primary.
For now, it is experimental and the syntax and semantics are subject to
change.