Python发送含附件邮件时，发件人含德语变音符号触发合规错误

问题场景 #

在Windows系统使用Python 3.11发送带附件邮件时，当发件人显示名（或邮箱地址）包含德语变音符号äöü，发送服务器会提示邮件不符合RFC 5321/5322标准；移除变音符号则可正常发送。相关最小复现代码如下：

import smtplib
import ssl
from email import encoders
from email.mime.base import MIMEBase
from email.mime.text import MIMEText
from email.mime.multipart import MIMEMultipart

message = MIMEMultipart()
message.set_charset('utf-8')
subject = 'Support-Nr. 1'
message["Subject"] = subject
body = 'testmail 123 äöüß'
message.attach(MIMEText(body, "plain"))
filename = 'VCARD_A & B.vcf'
with open(filename, "rb") as attachment:
    part = MIMEBase("application", "octet-stream")
    part.set_payload(attachment.read())
encoders.encode_base64(part)
part.add_header(
    "Content-Disposition",
    f"attachment; filename= {filename}",
)
message.attach(part)
sender_email = 'email1@example.com'
receiver_email = 'email2@example.com'
displayname = 'Höhne'
message["From"] = displayname + '<' + sender_email + '>'
message["To"] = receiver_email
smtplogin = # loginname
smtppassword = # passwort
smtpserver = 'smtp.ionos.de'
smtpport = 465
context = ssl._create_unverified_context()
with smtplib.SMTP_SSL(smtpserver, smtpport, context=context) as server:
    server.login(smtplogin, smtppassword)
    server.sendmail(sender_email, receiver_email, message.as_string())

问题原因 #

直接拼接显示名与邮箱地址作为From头，会让非ASCII字符（如ö）以原始UTF-8格式出现在邮件头中。而RFC 5321/5322明确要求，邮件头中的非ASCII字符必须按照RFC 2047标准进行编码（quoted-printable或Base64格式），否则服务器会判定邮件不合规。

解决方案 #

使用Python标准库email.utils中的formataddr函数构造From头，该函数会自动检测并编码非ASCII字符，确保邮件头符合RFC标准。

修改后的代码 #

import smtplib
import ssl
from email import encoders
from email.mime.base import MIMEBase
from email.mime.text import MIMEText
from email.mime.multipart import MIMEMultipart
# 新增导入
from email.utils import formataddr

message = MIMEMultipart()
message.set_charset('utf-8')
subject = 'Support-Nr. 1'
message["Subject"] = subject
body = 'testmail 123 äöüß'
message.attach(MIMEText(body, "plain"))
filename = 'VCARD_A & B.vcf'
with open(filename, "rb") as attachment:
    part = MIMEBase("application", "octet-stream")
    part.set_payload(attachment.read())
encoders.encode_base64(part)
# 可选：处理附件文件名的特殊字符编码
from email.header import encode_header
encoded_filename = encode_header(filename, 'utf-8').decode()
part.add_header(
    "Content-Disposition",
    f"attachment; filename= {encoded_filename}",
)
message.attach(part)
sender_email = 'email1@example.com'
receiver_email = 'email2@example.com'
displayname = 'Höhne'
# 替换原From头构造方式
message["From"] = formataddr((displayname, sender_email))
message["To"] = receiver_email
smtplogin = # loginname
smtppassword = # passwort
smtpserver = 'smtp.ionos.de'
smtpport = 465
context = ssl._create_unverified_context()
with smtplib.SMTP_SSL(smtpserver, smtpport, context=context) as server:
    server.login(smtplogin, smtppassword)
    server.sendmail(sender_email, receiver_email, message.as_string())

关键修改点 #

1. 导入email.utils.formataddr函数，用于合规构造发件人地址
2. 将原message["From"] = displayname + '<' + sender_email + '>'替换为message["From"] = formataddr((displayname, sender_email))
3. （可选）用email.header.encode_header处理附件文件名，避免特殊字符引发的编码问题

内容的提问来源于stack exchange，提问作者Erbs