Testing mail

Most tests never need a mail server. The double in mailkit/testkit satisfies mailkit.Sender and simply keeps what it is given:

sender := &testkit.Sender{}
service := NewInviteService(sender)

service.Invite(ctx, "maria@example.com")

if len(sender.Messages) != 1 {
  t.Fatal("no mail was sent")
}

Assert on sender.Messages, and set sender.Err to see how your code handles a relay being down. The double refuses exactly what a real sender refuses, an empty or malformed recipient and a cancelled context, so a test cannot pass on a message that would fail in production.

A fake relay for end to end tests

When you want the real SMTP wire, run go-smtp-mock inside the test and point the sender at it:

server := smtpmock.New(smtpmock.ConfigurationAttr{MultipleMessageReceiving: true})
server.Start()
defer server.Stop()

sender, err := smtp.New(smtp.Config{
  Host: "127.0.0.1",
  Port: server.PortNumber(),
  From: "crm@example.com",
  TLS:  smtp.TLSNone,
  HELO: "example.com",
})

Leaving PortNumber unset makes the fake pick a free port, which server.PortNumber() answers after Start. Read what arrived with server.WaitForMessages(1, timeout), and each message’s MsgRequest() is the full payload, headers and body together, ready for a substring assertion.

Three settings above are not decoration:

• MultipleMessageReceiving: true. go-mail resets the session after delivering, and without this flag the fake erases the recorded message when that happens.
• HELO: "example.com". The fake rejects the machine’s bare hostname, which is what go-mail announces by default.
• TLS: smtp.TLSNone and no credentials. The fake speaks neither encryption nor authentication, and mailkit refuses to send credentials without encryption anyway.