Emails sending inconsistently

[HeidiSeibold]

We’re currently running a virtual conference and have sent out emails through Indico to approximately 1000 people. Thank you for making this possible in the first place

Somehow the emails seem to be sent out in inconsistent batches and some fail and then there is a retry after a while, see this bit from the logs:

2025-08-01 03:42:15,727  WARNING  0000000000000000  indico.emails             Could not send email "[Indico] Instructions for joining the useR! 2025 virtual conference" (attempt 6/10); retry in 1800s [Connection unexpectedly closed]
2025-08-01 03:42:15,728  INFO     0000000000000000  celery.worker.strategy    Task send_email[b2376526-4a92-43db-b4ee-3132db6e6a9a] received
2025-08-01 03:42:15,730  INFO     0000000000000000  celery.worker.strategy    Task send_email[27612da9-6119-429a-a126-0261ce9cc58b] received
2025-08-01 03:42:15,732  INFO     0000000000000000  celery.worker.strategy    Task send_email[6fa851b5-7406-40f3-9148-19906879fc40] received
2025-08-01 03:42:15,733  INFO     0000000000000000  celery.worker.strategy    Task send_email[cb1165ee-da7e-4364-b0c2-55621574c42b] received
2025-08-01 03:42:15,734  INFO     0000000000000000  celery.worker.strategy    Task send_email[3eb75201-a569-40d7-89fc-eb912edf6cc9] received
2025-08-01 03:42:15,735  INFO     0000000000000000  celery.worker.strategy    Task send_email[3bbdf937-b2dc-4c2f-8a4d-11a05ba80b6f] received
2025-08-01 03:42:15,737  INFO     0000000000000000  celery.worker.strategy    Task send_email[8f4a96cf-4df9-4266-9a42-f28252cebb96] received
2025-08-01 03:42:15,738  INFO     0000000000000000  celery.worker.strategy    Task send_email[bbcbeec5-7372-4c37-bae2-f9b1e8c5cff3] received
2025-08-01 03:42:15,739  INFO     0000000000000000  celery.worker.strategy    Task send_email[04b957c7-8a5e-4598-bcee-c84cf3576e2d] received
2025-08-01 03:42:15,740  INFO     0000000000000000  celery.worker.strategy    Task send_email[11b73e6e-39fe-4511-8492-c39e2004d90c] received
2025-08-01 03:42:15,741  INFO     0000000000000000  celery.worker.strategy    Task send_email[906beebf-2d66-4c1b-85a7-4cf932341b20] received
2025-08-01 03:42:15,784  INFO     0000000000000000  celery.app.trace          Task send_email[906beebf-2d66-4c1b-85a7-4cf932341b20] retry: Retry in 1800s
2025-08-01 03:42:15,785  INFO     0000000000000000  celery.app.trace          Task send_email[11b73e6e-39fe-4511-8492-c39e2004d90c] retry: Retry in 1800s
2025-08-01 03:42:15,795  WARNING  0000000000000000  indico.emails             Could not send email "[Indico] Instructions for joining the useR! 2025 virtual conference" (attempt 6/10); retry in 1800s [Connection unexpectedly closed]
2025-08-01 03:42:15,796  INFO     0000000000000000  celery.worker.strategy    Task send_email[0e9af2a3-06f4-4448-bbf1-8b456d0a7142] received
2025-08-01 03:42:15,849  INFO     0000000000000000  celery.app.trace          Task send_email[0e9af2a3-06f4-4448-bbf1-8b456d0a7142] retry: Retry in 1800s
2025-08-01 03:42:15,896  WARNING  0000000000000000  indico.emails             Could not send email "[Indico] Instructions for joining the useR! 2025 virtual conference" (attempt 6/10); retry in 1800s [Connection unexpectedly closed]
2025-08-01 03:42:15,898  INFO     0000000000000000  celery.worker.strategy    Task send_email[3204cffb-46be-452b-98ce-6d842e1599b0] received
2025-08-01 03:42:15,931  WARNING  0000000000000000  indico.emails             Could not send email "[Indico] Instructions for joining the useR! 2025 virtual conference" (attempt 6/10); retry in 1800s [Connection unexpectedly closed]
2025-08-01 03:42:15,933  INFO     0000000000000000  celery.worker.strategy    Task send_email[e01b9d19-cbaf-4e3a-bfc2-25abcaa48803] received
2025-08-01 03:42:15,948  INFO     0000000000000000  celery.app.trace          Task send_email[3204cffb-46be-452b-98ce-6d842e1599b0] retry: Retry in 1800s
2025-08-01 03:42:15,956  WARNING  0000000000000000  indico.emails             Could not send email "[Indico] Instructions for joining the useR! 2025 virtual conference" (attempt 6/10); retry in 1800s [Connection unexpectedly closed]
2025-08-01 03:42:15,957  INFO     0000000000000000  celery.worker.strategy    Task send_email[bc6d8078-e174-4ae4-ab0c-8fd3472eac8e] received
2025-08-01 03:42:15,987  INFO     0000000000000000  celery.app.trace          Task send_email[e01b9d19-cbaf-4e3a-bfc2-25abcaa48803] retry: Retry in 1800s
2025-08-01 03:42:16,003  WARNING  0000000000000000  indico.emails             Could not send email "[Indico] Instructions for joining the useR! 2025 virtual conference" (attempt 6/10); retry in 1800s [Connection unexpectedly closed]
2025-08-01 03:42:16,004  INFO     0000000000000000  celery.worker.strategy    Task send_email[bbb3f9d5-01ef-4ea2-89fe-8286122bbd23] received
2025-08-01 03:42:16,013  INFO     0000000000000000  celery.app.trace          Task send_email[bc6d8078-e174-4ae4-ab0c-8fd3472eac8e] retry: Retry in 1800s
2025-08-01 03:42:16,060  INFO     0000000000000000  celery.app.trace          Task send_email[bbb3f9d5-01ef-4ea2-89fe-8286122bbd23] retry: Retry in 1800s
2025-08-01 03:42:16,095  WARNING  0000000000000000  indico.emails             Could not send email "[Indico] Instructions for joining the useR! 2025 virtual conference" (attempt 6/10); retry in 1800s [Connection unexpectedly closed]
2025-08-01 03:42:16,096  INFO     0000000000000000  celery.worker.strategy    Task send_email[0381c8fd-b14b-47db-bb0f-f8f2f01a0ce6] received
2025-08-01 03:42:16,109  WARNING  0000000000000000  indico.emails             Could not send email "[Indico] Instructions for joining the useR! 2025 virtual conference" (attempt 6/10); retry in 1800s [Connection unexpectedly closed]
2025-08-01 03:42:16,110  INFO     0000000000000000  celery.worker.strategy    Task send_email[27db89fb-4158-4415-86c8-5cab0887af84] received
2025-08-01 03:42:16,150  INFO     0000000000000000  celery.app.trace          Task send_email[0381c8fd-b14b-47db-bb0f-f8f2f01a0ce6] retry: Retry in 1800s
2025-08-01 03:42:16,167  INFO     0000000000000000  celery.app.trace          Task send_email[27db89fb-4158-4415-86c8-5cab0887af84] retry: Retry in 1800s
2025-08-01 03:42:16,282  WARNING  0000000000000000  indico.emails             Could not send email "[Indico] Instructions for joining the useR! 2025 virtual conference" (attempt 6/10); retry in 1800s [Connection unexpectedly closed]
2025-08-01 03:42:16,284  INFO     0000000000000000  celery.worker.strategy    Task send_email[1b68bbe6-ef29-4995-bab4-ce790ca099fb] received
2025-08-01 03:42:16,325  WARNING  0000000000000000  indico.emails             Could not send email "[Indico] Instructions for joining the useR! 2025 virtual conference" (attempt 6/10); retry in 1800s [Connection unexpectedly closed]
2025-08-01 03:42:16,326  INFO     0000000000000000  celery.worker.strategy    Task send_email[2482da4a-c789-4eeb-9d86-8a59e43d7add] received
2025-08-01 03:42:16,344  INFO     0000000000000000  celery.app.trace          Task send_email[1b68bbe6-ef29-4995-bab4-ce790ca099fb] retry: Retry in 1800s
2025-08-01 03:42:16,380  INFO     0000000000000000  celery.app.trace          Task send_email[2482da4a-c789-4eeb-9d86-8a59e43d7add] retry: Retry in 1800s
2025-08-01 03:42:16,399  WARNING  0000000000000000  indico.emails             Could not send email "[Indico] Instructions for joining the useR! 2025 virtual conference" (attempt 6/10); retry in 1800s [Connection unexpectedly closed]
2025-08-01 03:42:16,400  WARNING  0000000000000000  indico.emails             Could not send email "[Indico] Instructions for joining the useR! 2025 virtual conference" (attempt 6/10); retry in 1800s [Connection unexpectedly closed]
2025-08-01 03:42:16,400  INFO     0000000000000000  celery.worker.strategy    Task send_email[542b6b4b-30bf-4bcc-8729-475338d0db05] received
2025-08-01 03:42:16,402  INFO     0000000000000000  celery.worker.strategy    Task send_email[dcbf9b58-9608-4f77-aca3-5023de13e994] received
2025-08-01 03:42:16,428  WARNING  0000000000000000  indico.emails             Could not send email "[Indico] Instructions for joining the useR! 2025 virtual conference" (attempt 6/10); retry in 1800s [Connection unexpectedly closed]

I have the following questions:

1. Is Indico sending out emails in batches or does the batches have some other reason?
2. Is there a way for me to know why emails fail?
3. When an email is marked as sent (green envelope) in the web-logs, can we safely assume it really arrived?
   

Any help on any of these questions would be appreciated

Maybe relevant:

• We’re running Indico in a Docker container
• Emails are sent out via a Google Workspace email

[ThiefMaster]

@HeidiSeibold

Is Indico sending out emails in batches or does the batches have some other reason?

There’s no batching, Indico prepares the emails all at once, but since it goes through Celery tasks, they’ll be processed mostly sequentially (IIRC Celery uses one worker per CPU core by default, so likely 2-4 processes taking care of it on most setups).

@HeidiSeibold

Is there a way for me to know why emails fail?

“Connection unexpectedly close” is the error. so maybe the mail server thinks emails are being sent too fast and just cuts the connection? Maybe your mail server has more useful logs (or your mail provider in case you do not run the mail server yourself).

@HeidiSeibold

When an email is marked as sent (green envelope) in the web-logs, can we safely assume it really arrived?

That’s impossible to know. Green means that your mail server accepted it. Generally this means that it will try to deliver it to the recipient, and if it gets rejected, it generates a non-deliverability report to the sender address. That would usually be your noreply address so there’s a good chance you don’t see it unless you monitor it.

@HeidiSeibold

Emails are sent out via a Google Workspace email

I think there we may have the culprit. Unless GWS has some offering for programmatic mail sending, this - just like Gmail - is intended for humans sending emails. Not for any kind of bulk mailing. I’d absolutely not be surprised if Google has rate limits there and instead of cleanly reporting an error just cuts the connections if it thinks you’re spamming mailing too much…

[HeidiSeibold]

Thank you so much for the incredibly swift reply!

I think that helps us understand the issue better.

Now I guess we still have to Update to the new Indico version to allow me to limit which email addresses get to send emails (see Configuring Indico to Send from Single Email Address).

[ThiefMaster]

If you are not on 3.3.7 then yes, you should absolutely update. Also due to the security improvements in that release.