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SMTP and IMAP

When running the standalone version of Stalwart SMTP, it is possible to use an external SMTP, LMTP or IMAP server as a "virtual" directory. This allows you to delegate authentication and address validation to a third-party mail server. It is important to note that external SMTP or IMAP authentication cannot be used when running the all-in-one or JMAP servers.

Connection details​

The connection details for an external SMTP or IMAP server are specified under the directory.<name> key in the configuration file with the following attributes:

  • type: Specifies the type of remote server, which can be either imap, smtp or lmtp.
  • address: The address or hostname of the remote server.
  • port: The port number to connect to.
  • tls.implicit: When set to true, this setting means that Stalwart should use an implicitly secure connection to the server from the start of the connection. If it's false, it means that Stalwart should start the connection as a plain text connection, and then upgrade to a secure connection using the STARTTLS command (if available).
  • tls.allow-invalid-certs: This setting controls how Stalwart handles server certificates that it can't validate. If it's set to true, Stalwart will allow connections even if the server presents an invalid certificate. However, this is not recommended for production environments as it leaves the connection susceptible to man-in-the-middle attacks. It could be used for testing environments where a proper certificate isn't available.
  • limits.rcpt: The maximum number of recipients that can be tested in a single session.
  • limits.auth-errors: The maximum number of authentication errors that can occur in a single session before the connection is closed.

Example:

[directory."imap"]
type = "imap"
address = "imap.example.org"
port = 993

[directory."imap".tls]
implicit = true
allow-invalid-certs = false

[directory."lmtp"]
type = "lmtp"
address = "127.0.0.1"
port = 11200

[directory."imap".tls]
implicit = false
allow-invalid-certs = true

Domain lookup list​

Since external IMAP or SMTP servers are not able to validate which are the local domains that Stalwart SMTP is handling, it is necessary to specify a local domain lookup list. This is done under the directory.<name>.lookup.domains key in the configuration file. For example:

[directory."smtp".lookup]
domains = ["example.org", "example.com"]