This patch implements some measures against email loops, such as keeping
a limit on the lenght of an address, and rejecting email that has too
many Received headers.
It's not perfect (a server could be actively removing Received headers),
but it should cover the normal accidents and misconfigurations.
Having the certificates inside the domain directory may cause some confusion,
as it's possible they're not for the same name (they should be for the MX we
serve as, not the domain itself).
So it's not a problem if we have domains with no certificates (we could be
their MX with another name), and we could have more than one certificate per
"domain" (if we act as MXs with different names).
So this patch moves the certificates out of the domains into a new certs/
directory, where we do a one-level deep lookup for the files.
While at it, change the names of the files to "fullchain.pem" and
"privkey.pem", which match the names generated by the letsencrypt client, to
make it easier to set up. There's no general convention for these names
anyway.
This patch tidies up the Procmail courier:
- Move the configuration options to the courier instance, instead of using
global variables.
- Implement more useful string replacement options.
- Use exec.CommandContext for running the command with a timeout.
As a consequence of the first item, the queue now takes the couriers via its
constructor.
