Setup login accounts for sending mail

[jfly]

Relates to #485.

Emails on the nixos.org domain are mostly mailing lists. However, we occasionally enable people to send emails from nixos.org addresses.

• That isn't possible with the current mail-test.nixos.org infrastructure.
• @Mic92 tells me that we don't have to figure this out before launching the new mailserver, because currently the only use case for sending emails is elections@, which isn't needed right now (the SC election recently finished).
• We don't want to store incoming emails, or serve them over IMAP/pop3. Instead, we should just forward emails.
• This means we don't need dovecot. It's a little tricky to get rid of it, though:
  • It's not possible to disable dovecot with simple-nixos-mailserver (relevant code here: https://gitlab.com/simple-nixos-mailserver/nixos-mailserver/-/blob/af7d3bf5daeba3fc28089b015c0dd43f06b176f2/mail-server/dovecot.nix#L164)
  • simple-nixos-mailserver configures postfix to use dovecot for authentication: https://gitlab.com/simple-nixos-mailserver/nixos-mailserver/-/blob/af7d3bf5daeba3fc28089b015c0dd43f06b176f2/mail-server/postfix.nix#L250-253. We'd need to change this smtpd_sasl_path. This is documented by postfix here: https://www.postfix.org/SASL_README.html#server_cyrus
• Forwarding emails should be as simple as setting up a virtual_alias_maps file. We may want to encrypt these, so it may make sense to re-use the infrastructure in https://github.com/NixOS/infra/blob/master/non-critical-infra/modules/mailserver/mailing-lists.nix to do this.
  • This will require messing with the virtual_transport setting (see nixos-mailserver setting and the postfix docs). I'm not sure what exactly we'd set it to.
  • @Mic92 also mentioned the mailbox_transport setting, but I'm not sure it's relevant here.