MAIL stage

The MAIL FROM command initiates an SMTP message transfer by identifying the sender of the message. After the command is issued, the receiving mail server validates the sender address and checks whether the sender is authorised to send messages on behalf of that domain. If valid, the transaction proceeds to the RCPT TO stage.

MAIL-stage behaviour is configured on the MtaStageMail singleton (found in the WebUI under Settings › MTA › Session › MAIL FROM Stage).

Address rewriting

The rewrite field accepts an expression that can modify the sender address. This is useful for obfuscating the sender for privacy, or for changing the domain to match organisational branding. For background on the expression-based rewrite model see the address rewriting documentation.

For example, the following configuration removes any subdomain from the sender address on non-SMTP listeners:

{
  "rewrite": {
    "match": [{"if": "listener != 'smtp' & matches('^([^.]+)@([^.]+)\\.(.+)$', rcpt)", "then": "$1 + '@' + $3"}],
    "else": "false"
  }
}

Allowed senders

The isSenderAllowed field accepts an expression that determines whether the sender is accepted. If the expression evaluates to false, the sender is rejected. The default policy allows the sender when authenticated or when the sender domain is not present in the spam-block list.

For example, to block domains present in the spam-block list:

{
  "isSenderAllowed": {"else": "!key_exists('spam-block', sender_domain)"}
}

Sieve script

The script field selects a Sieve script to run after a successful MAIL FROM command. Typical uses include rejecting specific senders or rewriting the sender address using the envelope Sieve extension.

For example, setting script to the expression "'return_path_filter'" runs a Sieve script named return_path_filter which can reject messages whose return path matches a known-bad local part:

require ["variables", "envelope", "reject"];

if envelope :localpart :is "from" "known_spammer" {
    reject "Messages from this sender are not accepted.";
}