Python EmailMessage: keep received newlines out of a header field

Operation contract

The fixture tries to assign a received subject containing a newline and a second header name. EmailMessage rejects that assignment. It then sets a plain owned subject and body and serializes the message locally; nothing is sent.

Failure boundary

Header validation is one part of mail handling. Addresses, recipients, attachment types, message size, SMTP transport, and delivery rules need separate policies. The body of a message is not made trusted merely because its subject was accepted.

Working program

from email import policy
from email.message import EmailMessage

message = EmailMessage(policy=policy.SMTP)
try:
    message["Subject"] = "Receipt R-47 paid\nBcc: hidden"
except ValueError:
    print("newline_rejected", True)
message["Subject"] = "Receipt R-47 paid"
message.set_content("Receipt R-47 was recorded.")
serialized = message.as_bytes()
print("one_subject", serialized.count(b"Subject:") == 1)
print("body_present", b"Receipt R-47 was recorded." in serialized)

Output

newline_rejected True
one_subject True
body_present True

Costs and limits

Serialization allocates bytes proportional to the message body. This fixture keeps one short local message and makes no mail-delivery or attachment-budget claim.

Common Mistakes

• Do not concatenate a received value into raw header lines.
• A valid subject does not validate a recipient or body.
• Building bytes locally is not evidence that a message was delivered.

Connected lessons

• Python logging: allowlist fields before they reach a handler
• Python strings and bytes: reject decoding errors before parsing records
• Python JSON Lines: cap line bytes and validate the whole batch before returning it

Test this contract.