outbound_mail.toml:

-----------------------------------
[outbound_mail]
default_source_group = "fallback"  # Use mta4 for all other domains

[[outbound_mail.routing_rules]]
recipient_domain_pattern = "*.gmail.com"
source_group = "google"

[[outbound_mail.routing_rules]]
recipient_domain_pattern = "*.googlemail.com"
source_group = "google"

[[outbound_mail.routing_rules]]
recipient_domain_pattern = "*.outlook.com"
source_group = "outlook"

[[outbound_mail.routing_rules]]
recipient_domain_pattern = "*.hotmail.com"
source_group = "outlook"

[[outbound_mail.routing_rules]]
recipient_domain_pattern = "*.live.com"
source_group = "outlook"

[[outbound_mail.routing_rules]]
recipient_domain_pattern = "*.yahoo.com"
source_group = "yahoo"

[[outbound_mail.routing_rules]]
recipient_domain_pattern = "*.ymail.com"
source_group = "yahoo"

[[outbound_mail.routing_rules]]
recipient_domain_pattern = "*.rocketmail.com"
source_group = "yahoo"
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kumomta.toml
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[source_group."google"]
members = ["vmta1", "vmta2"]

[source_group."outlook"]
members = ["vmta3", "vmta4"]

[source_group."yahoo"]
members = ["vmta5", "vmta6"]

[source_group."fallback"]
members = ["vmta7", "vmta8", "vmta9", "vmta10", "vmta11", "vmta12"]
--------------------------------------------------------

init.lua:
----------------------------------------------------

kumo.on("get_outbound_source_group", function(txn)
  local rcpt_domain = txn:get("recipient_domain") or ""
  rcpt_domain = rcpt_domain:lower()

  kumo.log.info("Recipient domain: " .. rcpt_domain)

  if rcpt_domain:match("gmail%.com$") or rcpt_domain:match("googlemail%.com$") then
    kumo.log.info("Routing to group: google")
    return "google"
  elseif rcpt_domain:match("outlook%.com$") or rcpt_domain:match("hotmail%.com$") or rcpt_domain:match("live%.com$") then
    kumo.log.info("Routing to group: outlook")
    return "outlook"
  elseif rcpt_domain:match("yahoo%.com$") or rcpt_domain:match("ymail%.com$") then
    kumo.log.info("Routing to group: yahoo")
    return "yahoo"
  else
    kumo.log.info("Routing to group: fallback")
    return "fallback"
  end
end)
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