Invalidate SmtpClient connection when Host or Port changes

[rzikm]

Why

SmtpClient caches a live SmtpConnection in its transport and reuses it across Send calls. However, the Host and Port setters only cleared the legacy _servicePoint field and never dropped the cached connection. As a result, changing Host or Port between sends kept delivering mail to the original server instead of the newly configured one.

What

When Host or Port actually changes, release the cached transport connection so the next send re-establishes a fresh connection to the new target. Normal connection reuse (when the value is unchanged) is preserved, and the setters already throw while a send is in progress, so this never runs mid-send. ReleaseConnection is null-safe when no connection was ever opened.

Tests

Added regression tests in SmtpClientConnectionTest, which run across all three send paths (Send, SendAsync, SendMailAsync):

• ChangingPort_DoesNotReuseConnectionToPreviousServer - points the client at a second loopback server and asserts the next message reaches the new server and not the original.
• ChangingHost_EstablishesNewConnection - asserts a new connection is opened after the host changes.

Verified both tests fail without the fix and pass with it; the full System.Net.Mail functional suite (457 tests) passes.

Note

This PR was authored by GitHub Copilot.

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[Copilot]

Pull request overview

This PR updates System.Net.Mail.SmtpClient so that changing Host or Port invalidates any cached SMTP transport connection, ensuring subsequent sends connect to the newly configured endpoint rather than reusing a connection established for the previous host/port.

Changes:

• Release the cached transport connection when SmtpClient.Host changes.
• Release the cached transport connection when SmtpClient.Port changes.
• Add functional regression tests to verify host/port changes result in new connections and correct delivery.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
src/libraries/System.Net.Mail/src/System/Net/Mail/SmtpClient.cs	Releases the cached SMTP transport connection when Host/Port changes.
src/libraries/System.Net.Mail/tests/Functional/SmtpClientConnectionTest.cs	Adds regression tests ensuring host/port changes establish a new connection and deliver to the intended server.

[Copilot]

Pull request overview

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

[Copilot]

Pull request overview

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

Suppressed comments (1)

Previously missed (1) — in code that hasn't changed since the last review.

src/libraries/System.Net.Mail/src/System/Net/Mail/SmtpTransport.cs:23

• _stale is written from property setters (no lock) and read from IsConnected on the send path; without synchronization this can be lost/seen late across threads, causing an old connection to be reused even after an invalidating configuration change. Marking the field volatile makes the invalidation reliably observable without widening locking.

        private readonly SmtpClient _client;
        private ICredentialsByHost? _credentials;
        private bool _shouldAbort;
        private bool _stale;

[Copilot]

Pull request overview

Copilot reviewed 4 out of 4 changed files in this pull request and generated no new comments.

Suppressed comments (1)

Previously missed (1) — in code that hasn't changed since the last review.

src/libraries/System.Net.Mail/src/System/Net/Mail/SmtpTransport.cs:105

• GetConnectionAsync performs potentially blocking shutdown work (ReleaseConnection() / Abort()) while holding the SmtpTransport lock. This increases lock hold times across network I/O (QUIT) and can delay SmtpTransport.Abort() (used by the send timeout path). Consider clearing _connection under the lock, releasing/aborting the previous connection outside the lock, then reacquiring the lock to create the new connection (so Abort() during shutdown can still flow through _shouldAbort).

        {
            lock (this)
            {
                // Release any previously cached connection (for example one that became stale
                // after a configuration change, or one whose connect attempt failed) before

[Copilot]

Pull request overview

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.