Python3使用smtplib发邮件遇501行过长错误的解决咨询

问题场景 #

在Python3环境下使用smtplib库发送邮件时，仅在添加附件（哪怕是不到1MB的图片）或HTML内容过长的情况下，会触发SMTPDataError错误，错误码501，提示信息为“Syntax error - line too long”。

代码示例 #

# MSG building
msg = MIMEMultipart("alternative")
msg.attach(MIMEText(email_text, "plain"))
msg.attach(MIMEText(email_html, "html"))

msg["To"] = to_email
msg["From"] = from_email
msg["Subject"] = subject
msg["Date"] = formatdate()
msg["Message-ID"] = make_msgid(domain=sender_domain)

with open(attachment_dir, 'rb') as f:
    part = MIMEBase("application", "octet-stream")
    part.set_payload(f.read())
    
    # Encode to Base64                
    encoders.encode_base64(part)
    
    # Set mail headers
    part.add_header(
        "Content-Disposition", "attachment", filename=attachment_name
        )
    
    msg.attach(part)
    
# Python 3 libraries expect bytes.
msg_data = msg.as_bytes()

# Create secure connection with server and send email
context = ssl.create_default_context()

with smtplib.SMTP_SSL("smtp.ionos.co.uk", 465, context=context) as mailserver:
    mailserver.login(from_email, from_password)
    mailserver.sendmail(from_email, to_email, msg_data)

报错回溯 #

Traceback (most recent call last):
  File "/home/server/repos/mailsender/run.py", line 33, in <module>
    email.send()
  File "/home/server/repos/mailsender/main.py", line 139, in send
    mailserver.sendmail(from_email, to_email, msg_data)
  File "/usr/lib/python3.9/smtplib.py", line 892, in sendmail
    raise SMTPDataError(code, resp)
smtplib.SMTPDataError: (501, b'Syntax error - line too long')

核心原因 #

SMTP协议（RFC 2045）要求邮件内容的每行长度不得超过76个字符。当附件Base64编码后未按规范换行、或HTML文本未分段时，会出现单行过长的情况，触发服务器的语法错误。

语法层面解决方法 #

1. 规范附件的Base64编码换行 #

默认的encoders.encode_base64()可能存在换行不彻底的问题，手动对Base64编码后的内容按76字符分段换行：

import base64

# 替换原附件处理代码
with open(attachment_dir, 'rb') as f:
    part = MIMEBase("application", "octet-stream")
    # 读取文件并Base64编码
    raw_data = f.read()
    b64_data = base64.b64encode(raw_data).decode('utf-8')
    # 每76字符插入换行符
    wrapped_data = '\n'.join([b64_data[i:i+76] for i in range(0, len(b64_data), 76)])
    part.set_payload(wrapped_data)
    
    # 设置必要头部
    part.add_header("Content-Type", "application/octet-stream")
    part.add_header("Content-Transfer-Encoding", "base64")
    part.add_header(
        "Content-Disposition", "attachment", filename=attachment_name
    )
    
    msg.attach(part)

2. 处理过长的HTML内容 #

对长HTML文本使用quoted-printable编码，自动拆分长行：

# 替换原HTML内容添加代码
html_part = MIMEText(email_html, "html", "utf-8")
# 强制设置传输编码为quoted-printable
html_part.replace_header('Content-Transfer-Encoding', 'quoted-printable')
msg.attach(html_part)

3. 使用BytesGenerator生成合规邮件内容 #

替代msg.as_bytes()，用email.generator.BytesGenerator生成严格符合SMTP规范的邮件数据：

from email.generator import BytesGenerator
from io import BytesIO

# 生成合规的邮件字节流
buf = BytesIO()
generator = BytesGenerator(buf, maxheaderlen=76)
generator.flatten(msg, unixfrom=False)
msg_data = buf.getvalue()

内容的提问来源于stack exchange，提问作者Mr Growbot