Connection

A connection strategy defines how Stalwart MTA establishes SMTP connections to remote servers during message delivery. While routing strategies determine where messages are delivered, connection strategies control how those connections are made. A connection strategy specifies parameters such as the source IP address to bind to, the hostname to advertise in the EHLO command, and timeout values for each stage of the SMTP session. This level of control is useful in multi-homed systems, outbound IP rotation setups, or when complying with specific policy or network constraints.

Each connection strategy is defined as an MtaConnectionStrategy object (found in the WebUI under Settings › MTA › Outbound › Connection Strategies). Strategies are selected dynamically for each recipient via the connection expression on MtaOutboundStrategy; see Strategies for details.

Each connection strategy operates independently and can be tailored to specific domains, message types, or security requirements. For example, one strategy might use a dedicated IP address and hostname for high-volume transactional mail, while another uses different parameters for low-volume or internal mail.

EHLO hostname

The ehloHostname field overrides the EHLO hostname used when this strategy is applied. When left empty, the system hostname is used. For example:

{
  "name": "default",
  "ehloHostname": "mx.example.org"
}

Source IP

The sourceIps field accepts a list of local IPv4 and IPv6 addresses to use when delivering messages to remote servers. If multiple entries are provided, Stalwart randomly selects one for each new connection. Each entry carries a required sourceIp and an optional ehloHostname that overrides the strategy-wide EHLO hostname for that source address.

Correct configuration of sourceIps is important for deliverability on multi-homed systems. If no source address is set and the system has several local IPs, the kernel chooses one arbitrarily, which may hurt reputation or violate policy. Per-source EHLO overrides are useful when different IPs are associated with different domains or services.

Example strategy with four source addresses, one of which overrides the EHLO hostname:

{
  "name": "default",
  "ehloHostname": "mx.example.org",
  "sourceIps": [
    {"sourceIp": "192.0.2.10", "ehloHostname": "mx-192-0-2-10.example.org"},
    {"sourceIp": "192.0.2.11"},
    {"sourceIp": "2001:db8::a"},
    {"sourceIp": "2001:db8::b"}
  ]
}

Timeouts

Timeouts cap how long Stalwart waits for the remote server to respond at each stage of the SMTP dialogue: connectTimeout (connection establishment, default 5 minutes), greetingTimeout (SMTP greeting, default 5 minutes), ehloTimeout (EHLO response, default 5 minutes), mailFromTimeout (MAIL FROM response, default 5 minutes), rcptToTimeout (RCPT TO response, default 5 minutes), and dataTimeout (DATA response, default 10 minutes):

{
  "connectTimeout": "3m",
  "greetingTimeout": "3m",
  "ehloTimeout": "3m",
  "mailFromTimeout": "3m",
  "rcptToTimeout": "3m",
  "dataTimeout": "10m"
}