I'm using CPython 2.7 with the smtplib and email modules to send emails with SMTP.
Today, one of our clients complained that the email sent is not RFC 5322 compliant because the required Date header is missing. The RFC states in section 3.6.:

"The only required header fields are the origination date field and
 the originator address field(s).  All other header fields are
  syntactically optional."

Our program has been sending millions of email message this way and this is the first complaint.

I guess that the library doesn't add the header field automatically.