Python 3.13 breaks sending of emails

Wednesday, April 2, 2025

TIM has a single field PAR->email with the customer’s email addresses. And TIM users know that they can put multiple email addresses into this field if they separate them by a semicolon. A semicolon?! According to RFC 5322 the correct delimiter for recipients in the To: header is the comma, not a semicolon. Yes, but a semicolon works as well. Or at least it used to work as well. But some time ago (I guess in version 3.2) the Python email module has been modernized and no longer accepts strange values in the To: header.

TIM uses the sendmail.py script to actually send the emails.

>>> import email
>>> from email.utils import getaddresses

Here is an example of an email generated by TIM (the sendmail.py script reads this from a file and cares about the special cp850 encoding, but we can simplify this here):

>>> TEMPLATE = """From: foo@example.be
... To: {recipient}
... Subject: Rechnung
...
... Sehr geehrter Kunde,
... anbei unsere Rechnung mit Gesamtbetrag 407,90
... """

And here is a function to test how sendmail.py parses the recipients from the To: header:

>>> def test(to):
...     msg = email.message_from_string(TEMPLATE.format(recipient=to))
...     return getaddresses(msg.get_all('to'))

And yes, semicolons aren’t okay while commas are okay:

>>> test("bar@example.be;baz@example.be")
[('', '')]

>>> test("bar@example.be,baz@example.be")
[('', 'bar@example.be'), ('', 'baz@example.be')]

Until some time ago, the first example would also have returned a list of parser addresses. And now poof, more than a hundred customers of my customer didn’t get their invoice. Because two weeks ago my customer had inadvertently deleted the Python from their PC, and I had reinstalled it. The new version is 3.13 and I think that the old one was 3.10 but I’m not sure.

And I didn’t find a way to restore the old behaviour. The compat32 policy should do it (according to the docs) but doesn’t:

>>> from email.policy import compat32
>>> s = TEMPLATE.format(recipient="bar@example.be;baz@example.be")
>>> msg = email.message_from_string(s, policy=compat32)
>>> getaddresses(msg.get_all('to', []))
[('', '')]

I could customize the header_factory, but I’m afraid that this might change in the future.

So I’ll rather ask the TIM user to replace their semicolons by commas.

More precisely I wrote a TIM script for them:

// activate from command line using "tim /exec:actexec('parscan')"
DlgDefine("b",{||(PAR->email:=strtran(PAR->email,";",","))!=NIL})
Confirm(MsgSure())
DbfScan({oPar()},1,NIL,NIL,NIL,{||TryRecLock(DlgValue('b'))})
SetMsg(MsgDone())

More tests:

>>> getaddresses(["foo@example.com; bar@example.org"])
[('', '')]

>>> getaddresses(["foo@example.com; bar@example.org"], strict=False)
[('', 'foo@example.com'), ('', ''), ('', 'bar@example.org')]

>>> import sys
>>> sys.version
'3.12.3 (main, Feb  4 2025, 14:48:35) [GCC 13.3.0]'