Fix System.Net.Mail.Tests.SmtpClientTest.TestZeroTimeout (dotnet#116062)

Author: rzikm

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

@@ -19,6 +19,7 @@ internal sealed class SmtpTransport
         private SmtpConnection? _connection;
         private readonly SmtpClient _client;
         private ICredentialsByHost? _credentials;
+        private bool _shouldAbort;
 
         private bool _enableSsl;
         private X509CertificateCollection? _clientCertificates;
@@ -90,6 +91,11 @@ internal Task GetConnectionAsync<TIOAdapter>(string host, int port, Cancellation
             lock (this)
             {
                 _connection = new SmtpConnection(this, _client, _credentials, _authenticationModules);
+                if (_shouldAbort)
+                {
+                    _connection.Abort();
+                }
+                _shouldAbort = false;
             }
 
             if (NetEventSource.Log.IsEnabled()) NetEventSource.Associate(this, _connection);
@@ -146,7 +152,14 @@ internal void Abort()
         {
             lock (this)
             {
-                _connection?.Abort();
+                if (_connection != null)
+                {
+                    _connection.Abort();
+                }
+                else
+                {
+                    _shouldAbort = true;
+                }
             }
         }
     }