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pythonsmtpsmtplib

Line length limit in SMTP emails [Was: Message length limit sending HTML email with smtplib]

Here's a simple test that demonstrates the problem I'm seeing:

import smtplib
from email.message import EmailMessage

my_addr = 'my_email@my_domain.com'

str = ''
for i in range(500):
    str += f'{i}: <a href="https://google.com">https://google.com</a><br>'

msg = EmailMessage()
msg.set_content('')
msg.add_alternative(str, subtype='html')
msg['Subject'] = 'html test'
msg['From'] = my_addr
msg.add_header('reply-to', my_addr)
msg['To'] = my_addr

with smtplib.SMTP('localhost', timeout=30) as smtp_server:
    smtp_server.send_message(msg)

The resulting email seems to "break" about every 1000 bytes in the input html. This length includes any tags in the HTML string. The received email looks something like this:

enter image description here

enter image description here

enter image description here

I've tried different SMTP servers with different message length limits, but all were set to 10M or higher, so I doubt this had any effect. Also, I receive no error codes or exceptions. Everything sends cleanly. smtplib just seems to break up the message somehow.

Any idea what the problem is or how to correct this?

Solution

  • Problem:
    It is a result of line line length limit imposed by SMTP servers.
    It causes breaking too long lines.

    Solution:
    Insert line break characters into your html string.

    for i in range(500):
  str += f'{i}: <a href="https://google.com">https://google.com</a><br>\n'

    tidy program may be used to break long HTML lines (among other things).

    Links:
    Is there an accepted maximum line length for SMTP header fields?