Fix SMTP connection from_email being ignored by email_on_failure and …

[bramhanandlingala]

Summary

_send_error_email_notification (used for email_on_failure / email_on_retry) resolved [email] from_email before constructing SmtpNotifier, so SmtpNotifier's own connection-extra fallback branch never ran. This meant the SMTP connection's from_email extra was silently ignored whenever [email] from_email was set — or even when it wasn't, since a hardcoded airflow@airflow fallback was passed in regardless.

This PR moves from_email precedence resolution into SmtpNotifier itself:

1. Explicit from_email argument (if passed directly to SmtpNotifier)
2. SMTP connection extra from_email
3. [email] from_email config
4. Hardcoded airflow@airflow default

task_runner.py now passes from_email=None to SmtpNotifier and lets it resolve precedence, instead of resolving [email] from_email upfront and locking out the connection-extra check.

Root cause

# before (task_runner.py)
from_email=conf.get("email", "from_email", fallback="airflow@airflow"),

Since this was never None, SmtpNotifier._build_email_content's if self.from_email is None: branch — which checks the SMTP connection's from_email extra — could never execute.

Fix

• providers/smtp/src/airflow/providers/smtp/notifications/smtp.py: _build_email_content now falls back through connection extra → [email] from_email config → hardcoded default, instead of raising when the connection extra is unset.
• task-sdk/src/airflow/sdk/execution_time/task_runner.py: passes from_email=None so the notifier's own resolution logic runs.

Testing

• Added 3 new unit tests in test_smtp.py covering all precedence combinations (connection-only, config-only, connection-over-config, neither-set-default).
• Updated 3 existing assertions in test_task_runner.py to reflect that from_email resolution now happens inside SmtpNotifier, not task_runner.py.

Fixes #69262

[SameerMesiah97]

Only one clarification. Looks good otherwise.

[SameerMesiah97]

Are there any other callers that still resolve from_email themselves before constructing the notifier? Please check.

[bramhanandlingala]

Hi @SameerMesiah97

Good question — I checked. SmtpNotifier is only constructed in one production call site outside its own module, which is _send_error_email_notification in task_runner.py, and that's already updated in this PR to pass from_email=None. I didn't find any other operator or utility in the repo that resolves from_email from config before building the notifier, so this should cover all callers.

[stephsi]

Hello @bramhanandlingala!

Thanks for creating a PR related to my issue. I think your suggested changes make sense overall.

I have one suggestion though:
Your three test cases now cover config-only, connection-over-config and neither-set-default , I would argue that you should also adjust the already existing test case connection-only to the style of your three created test cases, so it is more clear that these four test cases belong together. Also add a comment at the beginning to the already existing test case similar to your three test cases.

I also have some wording suggestions for the test comments, but these are maybe more personal preferences.

[stephsi]

Suggested change

	"""When from_email is unset and the connection defines none, `[email] from_email` config is used."""
	"""When config [email] from_email is set and the SMTP connection extra `from_email` is NOT set, `[email] from_email` config is used."""

I assume unset was a typing mistake as this is the test case where the config is set and the connection extra field is not set.

[bramhanandlingala]

Hi @stephsi

Good catch, thanks — that was indeed a typo, "unset" should've said the connection extra is not set. Fixed in the latest commit using your suggested wording exactly. I've also added a matching docstring to test_notifier_with_nondefault_connection_extra so all four tests (connection-only, config-only, connection-over-config, neither-set-default) are now consistent in style, per your other comment. Happy to take any further wording suggestions if you have them.

[bramhanandlingala]

Thanks @potiuk

@stephsi, @bugraoz93, @choo121600, @ephraimbuddy, @henry3260, @jason810496, @rawwar, @kaxil, @ashb, @amoghrajesh, @Lee-W
Request You All, please review and Merge as need for this fix