503 Sender already specified error when sending campaign

[roshanjonah]

I am getting this rather weird error saying "503 Sender already specified."

It has been mentioned in #207 as well. Here are my settings and I am using AWS SES to send.

It only seems to send to the very first recipient and getting these errors right after. I don't get any errors when the campaign starts or finishes.

The two subscriber UUID mentioned there are:
contact@privatedomain.com
username@gmail.com

[roshanjonah]

1. Potential solution? https://community.perforce.com/s/article/341 - Maybe Listmonk is sending more than 1 per connection?
2. Or maybe it's trying to specify the sender rather than continuing on with the next recipient?

@knadh Any ideas on how I may be able to limit it to 1 email per connection? I could only find the setting for limiting concurrent max connections. Thank you! =)

[knadh]

Maybe Listmonk is sending more than 1 per connection?

It indeed is and that's normal. It uses SMTP connection pooling / keep-alive. If you'd to throw away and open new connections for every e-mail, the throughput would be abysmal.

Or maybe it's trying to specify the sender rather than continuing on with the next recipient?

A rare race condition in the SMTP lib, possible. But an issue that triggers every time with just a handful or recipients, highly unlikely, as there are many production instances of listmonk that process large volumes, including ones that use SES.

Can you try reducing message rate, concurrency, and idle timeout (to say 3s) and see if that makes a difference?

[roshanjonah]

Yea that makes sense but it should be okay even with multiple emails via each connection.

I tried setting message rate to 1, concurrency to 1, and idle timeout to 3s, and max connections to 1 for AWS SMTP server but still end up saying the same error. I tried with other types of SMTP and they work fine - no issues there. Just with AWS SMTP type. Its so weird :(

@knadh You are right - this seems to occur randomly and only for smaller test lists - it would email two and the next two/one it would put it as this error.

[roshanjonah]

If we comment arbitrary messages or do this, then it seems to work! I get all the emails or else it would send 4 emails successfully and for one of them it generates the 503 error.

[knadh]

Circling back, did you test this with Max message retries set to 1?

[roshanjonah]

Yup tried that too mate but just didn't seem to work. Now we switched to using API so it uses SendRawEmail of AWS to send email through. But when we were using SMTP - it was doing that.

[citrus-it]

I've been debugging this exact symptom on my new listmonk installation and it turned out to be that I had not configured any email addresses for 'Admin notification emails'. That was causing an arbitrary message to be enqueued (campaign completion) without a To address and when it got processed it, nothing after it would be sent.

[knadh]

That was causing an arbitrary message to be enqueued (campaign completion) without a To address and when it got processed it, nothing after it would be sent.

Ah, well spotted! It shouldn't be attempting to e-mail if no address is available. Will fix this.

[Jay991]

I've been debugging this exact symptom on my new listmonk installation and it turned out to be that I had not configured any email addresses for 'Admin notification emails'. That was causing an arbitrary message to be enqueued (campaign completion) without a To address and when it got processed it, nothing after it would be sent.

I have a email address for 'Admin notification emails' and still getting this error ' 503 Only one sender per message,'

[mariojfduarte]

same here