smtpclient: handle server closing connection after writing its response to RCPT TO

if icloud.com has your ip blocklisted, it will close the smtp connection after
writing a response to RCPT TO, before writing a response to a pipelined DATA
command. this is similar to the case (already handled) where a mail server
would close the connection after a response to MAIL FROM when pipelined.

we now recognize this situation (unexpected EOF before we get a response to
DATA, with all RCPT TO's failed), and treat the last response to RCPT TO as the
result.

for issue #198 by soheilpro, thanks for reporting and sending an smtpclient
trace that showed the behaviour.

smtpclient/client.go

@@ -1265,6 +1265,15 @@ func (c *Client) DeliverMultiple(ctx context.Context, mailFrom string, rcptTo []
 			c.xbotchf(0, "", "", nil, "writing pipelined mail/rcpt/data: %w", writeerr)
 		}
 
+		// If remote closed the connection before writing a DATA response, and the RCPT
+		// TO's failed (e.g. after deciding we're on a blocklist), use the last response
+		// for a rcptto as result.
+		if dataerr != nil && errors.Is(dataerr, io.ErrUnexpectedEOF) && nok == 0 {
+			c.botched = true
+			r := rcptResps[len(rcptResps)-1]
+			c.xerrorf(r.Permanent, r.Code, r.Secode, r.Line, r.MoreLines, "%w: server closed connection just before responding to data command", ErrStatus)
+		}
+
 		// If the data command had an i/o or protocol error, it's also a failure for the
 		// entire transaction.
 		if dataerr != nil {

smtpclient/client_test.go

@@ -758,6 +758,33 @@ func TestErrors(t *testing.T) {
 		}
 	})
 
+	// Remote closes connection after 554 response to RCPT TO in pipelined
+	// connection. Should result in permanent error, not temporary read error.
+	// E.g. icloud.com that has your IP blocklisted.
+	run(t, func(s xserver) {
+		s.writeline("220 mox.example")
+		s.readline("EHLO")
+		s.writeline("250-mox.example")
+		s.writeline("250-ENHANCEDSTATUSCODES")
+		s.writeline("250 PIPELINING")
+		s.readline("MAIL FROM:")
+		s.writeline("250 2.1.0 ok")
+		s.readline("RCPT TO:")
+		s.writeline("554 5.7.0 Blocked")
+	}, func(conn net.Conn) {
+		c, err := New(ctx, log.Logger, conn, TLSOpportunistic, false, localhost, zerohost, Opts{})
+		if err != nil {
+			panic(err)
+		}
+
+		msg := ""
+		err = c.Deliver(ctx, "postmaster@other.example", "mjl@mox.example", int64(len(msg)), strings.NewReader(msg), false, false, false)
+		var xerr Error
+		if err == nil || !errors.Is(err, ErrStatus) || !errors.As(err, &xerr) || !xerr.Permanent {
+			panic(fmt.Errorf("got %#v, expected ErrStatus with Permanent", err))
+		}
+	})
+
 	// If we try multiple recipients and first is 452, it is an error and a
 	// non-pipelined deliver will be aborted.
 	run(t, func(s xserver) {