Dovecot

Dovecot is a mail storage server. It handles the storage of e-mail messages and their retrieval by a e-mail client (mail user agent) via authenticated IMAP.

This article describes a basic Dovecot setup with Postfix and virtual users, i.e., e-mail users are configured separately in Dovecot passdb, and are independent from system users.

SSL Certificate with ACME

This article assumes a working ACME configuration for certificate renewal.

{
  config,
  ...
}:
let
  sslCertDir = config.security.acme.certs."example.org".directory;
  domainName = "example.org";
in
{
  # further dovecot configuration goes here
  ...
}

Dovecot passwd database

Create a plaintext virtual user password database in the following format

  # authenticate via Passwd-file
  #
  # bill@example.org:{PLAIN}your_plain_password
  # alice@example.org:{PLAIN-MD5}1a1dc91c907325c69271ddf0c944bc72::::::
  # This file need to have correct permissions.

  age.secrets.dovecot = {
    file = "./dovecot-secret.age";
    # -rw-------
    mode = "600";
    owner = "dovecot2";
    group = "dovecot2";
  };

Open firewall port

Open firewall port for IMAPS clients.

  networking.firewall.allowedTCPPorts = [ 993 ]; # dovecot imaps

Setup auth via passwd-file

  services.dovecot2 = {
    enable = true;

    # use my own passwd file for auth, see below
    enablePAM = false;

    # https://doc.dovecot.org/2.3/configuration_manual/howto/postfix_dovecot_lmtp/
    # https://doc.dovecot.org/2.3/configuration_manual/howto/postfix_and_dovecot_sasl/
    extraConfig = ''
      # force to use full user name plus domain name
      # for disambiguation
      auth_username_format = %Lu

      # Authentication configuration:
      auth_mechanisms = plain
      passdb {
        driver = passwd-file
        args = ${config.age.secrets.dovecot.path}
      }
    ''
  };

Setup virtual users

  # /var/spool/mail/vmail needs to be created and owned by vmail
  users.users."vmail" = {
    createHome = true;
    home = "/var/spool/mail/vmail";
  };

  services.dovecot2 = {
    # configure virtual mail user and group
    createMailUser = true;
    mailUser = "vmail";
    mailGroup = "vmail";

    # implement virtual users
    # https://doc.dovecot.org/2.3/configuration_manual/howto/simple_virtual_install/
    # store virtual mail under
    # /var/spool/mail/vmail/<DOMAIN>/<USER>/Maildir/
    mailLocation = "maildir:~/Maildir";

    extraConfig = ''
      userdb {
        driver = static
        # the full e-mail address inside passwd-file is the username (%u)
        # user@example.com
        # %d for domain_name %n for user_name
        args = uid=vmail gid=vmail username_format=%u home=/var/spool/mail/vmail/%d/%n
      }
    '';
  };

Connect to postfix via lmtp

See Postfix for further setup on postfix side.

  services.dovecot2 = {
    # connection to postfix
    enableLmtp = true;

    # https://doc.dovecot.org/2.3/configuration_manual/howto/postfix_dovecot_lmtp/
    # https://doc.dovecot.org/2.3/configuration_manual/howto/postfix_and_dovecot_sasl/
    extraConfig = ''
      # connection to postfix via lmtp
      service lmtp {
       unix_listener /var/spool/postfix/dovecot-lmtp {
         mode = 0600
         user = postfix
         group = postfix
        }
      }
      service auth {
        unix_listener /var/spool/postfix/auth {
          mode = 0600
          user = postfix
          group = postfix
        }
      }
    '';
  };

  # postfix connection to dovecot
  services.postfix.config = {
    # https://doc.dovecot.org/2.3/configuration_manual/howto/postfix_dovecot_lmtp/
    mailbox_transport = "lmtp:unix:/var/spool/postfix/dovecot-lmtp";
    virtual_transport = "lmtp:unix:/var/spool/postfix/dovecot-lmtp";

    # https://doc.dovecot.org/2.3/configuration_manual/howto/postfix_and_dovecot_sasl/
    smtpd_sasl_type = "dovecot";
    smtpd_sasl_path = "/var/spool/postfix/auth";
    smtpd_sasl_auth_enable = "yes";
    smtpd_recipient_restrictions = "permit_mynetworks, permit_sasl_authenticated, reject_unauth_destination";
  };

  # create path for dovecot socket
  users.users."postfix" = {
    createHome = true;
    home = "/var/spool/postfix";
  };

Configure SSL Certificates

  services.dovecot2 = {
    sslServerCert = "${sslCertDir}/fullchain.pem";
    sslServerKey = "${sslCertDir}/key.pem";
    sslCACert = "${sslCertDir}/chain.pem";
  };

You also need to set proper file permissions on the cert directory and key files. See ACME#Integration_with_service_modules.

mail_crypt plugin (encryption at rest)

The following seems to make mail_crypt work in its per-user/per-folder mode (note that this mode is still described as 'not production quality' in the dovecot docs):

security.pam.services.dovecot2 = { }; # needed as we disable PAM below

services.dovecot2 = {
  enable = true;
  enablePAM = false; # need to disable this as we redefine passdb
  mailPlugins.globally.enable = [ "mail_crypt" ]; 
  pluginSettings = {
    mail_crypt_curve = "secp521r1";
    mail_crypt_save_version = "2";
    mail_crypt_require_encrypted_user_key = "yes";
  }; 
  extraConfig = '' 
    mail_attribute_dict = file:%h/.attributes
    userdb {
      driver = passwd
    }  
    passdb {
      driver = pam
      override_fields = userdb_mail_crypt_private_password=%{sha256:password} userdb_mail_crypt_save_version=2
      args = failure_show_msg=yes dovecot2
    }  
  '';
}; 

Troubleshooting

sievec fails to compile basic sieve scripts

Sieve commands such as fileinto need to be enabled explicitly with:

services.dovecot2.sieve.globalExtensions = ["fileinto"];

Otherwise, the sievec command will fail to compile sieve scripts with fileinto statements and as a result the Dovecot service itself will fail to start if the configuration contains services.dovecot2.sieve.scripts.