Nim std/smtp连接web.de发邮件报503 Bad sequence of commands错误

问题背景 #

我使用Nim编程语言开发Web应用，近期计划实现邮件发送相关功能。Nim官方提供的std/smtp标准库附带了starttls和ssl两种连接方式的简易使用示例，我在本地邮件服务商web.de注册了测试邮箱用于功能调试，尽可能严格参考示例编写代码后始终无法正常运行。

测试代码 #

import std/[smtp]

let smtpServerName = "smtp.web.de"
let startTlsportNumber = 587
let un = "dummymail" # 替换为实际用户名
let pw = "dummypw" # 替换为实际密码
let target = "bla@mailinator.com"

# StartTLS 代码块：两种模式二选一启用，不要同时生效
let smtpConn = newSmtp(debug=true)
smtpConn.connect(smtpServerName, Port startTlsportNumber)
smtpConn.startTls()


# SSL 代码块：两种模式二选一启用，不要同时生效
# let sslPortNumber = 465
# let smtpConn = newSmtp(useSsl = true, debug=true)
# smtpConn.connect(smtpServerName, Port sslPortNumber)


var msg = createMessage(mSubject = "Hello from Nim's SMTP",
                        mBody = "Hello!.\n Is this awesome or what?",
                        mTo = @[target])
smtpConn.auth(un, pw)
smtpConn.sendmail(un, @[target], $msg)

报错现象 #

我分别测试了两种连接模式，均触发运行时错误：

• 587端口startTLS模式：调用smtpConn.startTls方法时，服务器返回503响应，调试日志如下：

S:220 web.de (mrweb105) Nemesis ESMTP Service ready
C:HELO smtp.web.de

S:250 web.de Hello smtp.web.de [46.183.103.17]
C:STARTTLS

S:503 Bad sequence of commands
C:QUIT
/home/philipp/dev/playground/src/playground.nim(11) playground
/usr/lib/nim/pure/smtp.nim(246) startTls
/usr/lib/nim/pure/smtp.nim(226) checkReply
/usr/lib/nim/pure/smtp.nim(110) quitExcpt
Error: unhandled exception: Expected 220 reply, got: 503 Bad sequence of commands [ReplyError]

• 465端口SSL模式：调用smtpConn.connect方法时就收到服务器返回的503响应，调试日志如下：

S:220 web.de (mrweb005) Nemesis ESMTP Service ready
C:HELO smtp.web.de

S:503 Bad sequence of commands
C:QUIT
/home/philipp/dev/playground/src/playground.nim(15) playground
/usr/lib/nim/pure/smtp.nim(240) connect
/usr/lib/nim/pure/smtp.nim(231) helo
/usr/lib/nim/pure/smtp.nim(226) checkReply
/usr/lib/nim/pure/smtp.nim(110) quitExcpt
Error: unhandled exception: Expected 250 reply, got: 503 Bad sequence of commands [ReplyError]

已排查内容 #

• 端口配置核对无误：服务商明确说明587为startTls端口、465为SSL端口
• 编译参数无问题：在Arch Linux环境下编译时均添加了-d:ssl参数，排除SSL相关编译参数缺失问题
• 同一份代码连接Gmail邮箱可正常发送邮件，仅连接web.de时始终失败
• 检索相关错误信息提示该问题可能与认证有关，但按照SMTP协议流程，认证应在startTls建立加密连接之后执行，未排查出std/smtp库的使用错误，咨询该问题的根因与解决方案。

内容的提问来源于stack exchange，提问作者Philipp Doerner