SMTP

builder.Services.AddSmtpMailTransport();
builder.Services.AddPickupDirectoryMailTransport();

Neither call takes configuration: everything a transport needs arrives on the account, so one registration serves any number of them. Like the rest of the library, this package has no ASP.NET Core dependency, so a worker or a console host can use it.

Settings

Auto is the default and also the zero value, so a setting that is missing can never leave a session unencrypted. A value that is not a member name — STARTTLS, say, or a typo — is refused rather than falling back, for the same reason.

What goes on the wire

A connection, a handshake and an authentication per message. That is the honest cost of sending one mail, and it is what keeps the transport stateless.

Only From is set, never Sender: setting both makes the envelope and the header disagree, which is what breaks DMARC alignment. When a message sets EnvelopeFrom, that address is used for SMTP MAIL FROM while From stays what the recipient sees — which is how you send as a customer's domain and keep bounces, and the SPF record they align with, on your own.

Priority is written to X-Priority, which has five levels, and folded onto Importance, which has three and is what most clients actually display.

Headers the message composes itself — To, Cc, Bcc, From, Subject, Return-Path, Resent-* and the rest — cannot be set through EmailMessage.Headers. That is a security boundary, not tidiness: MimeKit merges a Bcc header into the real recipient list, so a host forwarding a user-supplied header bag would otherwise be handing out blind copies of its own mail.

Certificates

SmtpMailTransport.ValidateServerCertificate accepts only a certificate with nothing wrong with it. It is protected virtual, so a host that must pin a self-signed development certificate can override it and register the subclass instead of calling AddSmtpMailTransport().

The pickup directory

"local": {
  "Transport": "pickup",
  "DefaultFrom": "no-reply@example.com",
  "Settings": { "Directory": "./mail-drop" }
}

Each message is written as a real .eml, which opens in any mail client — so an HTML body renders exactly as the recipient would see it, including inline images. Useful for development, for a test that wants to look at the whole message, and as the drop point a local relay agent consumes.

The file name is generated, never derived from the subject or an address. The message is written under a temporary name and moved into place, so an agent watching the directory never reads half of one.

In memory

The core package's MemoryMailTransport keeps messages instead of delivering them, for a test that asserts on what would have been sent:

services.AddMemoryMailTransport();

// ... after the send
var sent = Assert.Single(provider.GetRequiredService<MemoryMailTransport>().GetSentMessages());

One instance is registered under both IMailTransport and its own type, so what a send writes to is what you read back. Accounts reach it with "Transport": "memory".