some linter nags checked and resolved.

[d-fal]

there are many linter errors that should be resolved. I checked some, there are some around.

client_test.go:524:5: `sendMailServer` is unused (deadcode)
var sendMailServer = `220 hello world
    ^
client_test.go:534:5: `sendMailClient` is unused (deadcode)
var sendMailClient = `EHLO localhost
    ^
client.go:454:22: Error return value of `d.c.conn.SetDeadline` is not checked (errcheck)
        d.c.conn.SetDeadline(time.Now().Add(d.c.SubmissionTimeout))
                            ^
client_test.go:42:8: Error return value of `c.Auth` is not checked (errcheck)
        c.Auth(toServerEmptyAuth{})
              ^
client_test.go:256:12: Error return value of `pw.Write` is not checked (errcheck)
                        pw.Write([]byte(l))
                                ^
client_test.go:635:15: Error return value of `c.Quit` is not checked (errcheck)
                defer c.Quit()
                            ^
client_test.go:671:11: Error return value of `s.w.Write` is not checked (errcheck)
        s.w.Write([]byte(f + "\r\n"))
                 ^
conn.go:184:28: Error return value of `c.bdatPipe.CloseWithError` is not checked (errcheck)
                c.bdatPipe.CloseWithError(ErrDataReset)
                                         ^
conn.go:189:19: Error return value of `c.session.Logout` is not checked (errcheck)
                c.session.Logout()
                                ^
conn.go:615:17: Error return value of `session.Logout` is not checked (errcheck)
                session.Logout()
                              ^
conn.go:655:9: Error return value of `io.Copy` is not checked (errcheck)
        io.Copy(ioutil.Discard, r) // Make sure all the data has been consumed
               ^
conn.go:695:10: Error return value of `io.Copy` is not checked (errcheck)
                io.Copy(ioutil.Discard, io.LimitReader(c.text.R, int64(size)))
                       ^
conn.go:717:22: Error return value of `r.CloseWithError` is not checked (errcheck)
                                        r.CloseWithError(errPanic)
                                                        ^
conn.go:737:20: Error return value of `r.CloseWithError` is not checked (errcheck)
                        r.CloseWithError(err)
                                        ^
conn.go:748:10: Error return value of `io.Copy` is not checked (errcheck)
                io.Copy(ioutil.Discard, chunk)
                       ^
conn.go:947:26: Error return value of `c.conn.SetWriteDeadline` is not checked (errcheck)
                c.conn.SetWriteDeadline(time.Now().Add(c.server.WriteTimeout))
                                       ^
conn.go:963:20: Error return value of `c.text.PrintfLine` is not checked (errcheck)
                c.text.PrintfLine("%d-%v", code, text[i])
                                 ^
conn.go:966:20: Error return value of `c.text.PrintfLine` is not checked (errcheck)
                c.text.PrintfLine("%d %v", code, text[len(text)-1])
                                 ^
conn.go:968:20: Error return value of `c.text.PrintfLine` is not checked (errcheck)
                c.text.PrintfLine("%d %v.%v.%v %v", code, enhCode[0], enhCode[1], enhCode[2], text[len(text)-1])
                                 ^
conn.go:988:28: Error return value of `c.bdatPipe.CloseWithError` is not checked (errcheck)
                c.bdatPipe.CloseWithError(ErrDataReset)
                                         ^
server.go:124:18: Error return value of `s.handleConn` is not checked (errcheck)
                go s.handleConn(newConn(c, s))
                               ^
lmtp_server_test.go:16:16: Error return value of `io.WriteString` is not checked (errcheck)
        io.WriteString(c, "MAIL FROM:<[email protected]>\r\n")
                      ^
lmtp_server_test.go:18:16: Error return value of `io.WriteString` is not checked (errcheck)
        io.WriteString(c, "RCPT TO:<[email protected]>\r\n")
                      ^
lmtp_server_test.go:20:16: Error return value of `io.WriteString` is not checked (errcheck)
        io.WriteString(c, "RCPT TO:<[email protected]>\r\n")
                      ^
server_test.go:104:12: Error return value of `io.CopyN` is not checked (errcheck)
                        io.CopyN(ioutil.Discard, r, s.backend.dataErrOffset)
                                ^
server_test.go:171:12: Error return value of `s.Serve` is not checked (errcheck)
        go s.Serve(l)
                  ^
server_test.go:759:12: Error return value of `s.Serve` is not checked (errcheck)
        go s.Serve(l)
                  ^
example_test.go:64:1: tests: ExampleSendMail_PlainAuth refers to unknown field or method: SendMail.PlainAuth (govet)
func ExampleSendMail_PlainAuth() {
^
backendutil/transform_test.go:97:59: Error return value of `pw.CloseWithError` is not checked (errcheck)
        go copyAndClose(w, r, func(err error) { pw.CloseWithError(err) })
                                                                 ^
backendutil/transform_test.go:127:12: Error return value of `s.Serve` is not checked (errcheck)
        go s.Serve(l)