Fix task error email notification sender fallback to connection and provider defaults - #72745

Closing this. [email] from_email defaults to None, so with this change the default SmtpNotifier path gets from_email=None, and SmtpNotifier raises a ValueError ("You should provide from_email or define it in the connection") unless the SMTP connection sets one. Failure and retry emails that go out from airflow@airflow today would stop being sent in a default setup. The tests only assert that from_email is None.

I understand your point and the PR can be reworked, but why it was just closed?
@olayiwola-gbenga Can you please provide here the business need for this change?

Thanks for the review @kaxil, and thanks @VladaZakharova for following up.

On the business need: in Airflow 2 (airflow.utils.email.send_email), from_email defaulted to None when [email] from_email wasn't set in airflow.cfg. Many users migrating to Airflow 3 use a custom [email] email_backend (like SendGrid or Amazon SES, which now go through _LegacyEmailBackendNotifier) and leave [email] from_email unset so the backend can fall back to its own sender config (such as the SENDGRID_MAIL_FROM env var or connection extras).
Because _send_error_email_notification currently passes "airflow@airflow" as the fallback, _LegacyEmailBackendNotifier passes "airflow@airflow" instead of None to the custom send_email function. That overrides the provider's own default sender and causes task failure/retry emails to be rejected due to an unverified sender address.

@kaxil makes a good point about SmtpNotifier. When I started working on this, there was an open PR for #69262 that updated SmtpNotifier to fall back to "airflow@airflow" instead of raising a ValueError when from_email=None, while leaving _LegacyEmailBackendNotifier with the hardcoded "airflow@airflow". Since that PR was closed without merging, passing from_email=None to SmtpNotifier currently raises a ValueError in a default setup when the SMTP connection doesn't have from_email set.

Since the scope of our ticket is specifically to fix legacy/custom email_backend setups, I can update this PR so fallback=None is only used for _LegacyEmailBackendNotifier while keeping fallback="airflow@airflow" for SmtpNotifier, and update the tests accordingly.

@kaxil could we reopen this PR so I can push that change?

Happy to reopen it @olayiwola-gbenga . But do ensure you follow PR guidelines, and show enough evidence of testing with before/after.

The current influx of PRs is not maintainable from the PR review standpoint, so any PRs that don't provide enough details, or are clearly AI slop -- will be closed without any further explanation.