We started getting error Expected response code "250" but got empty code.
when sending emails via Symfony Mailer on Laravel 10 and Mailchimp.
There are lots of similar questions and all of them were related to invalid credentials/not refreshed configs/etc. E.g. something was wrong with the setup.
In our case it is different.
Emails are sent by Laravel queue workers with retries. And when this error happens the email might be resent successfully on retry by the same worker on the same server.
So it is not a config issue.
This is how successful retry looks in logs:
But what else could cause the same error?
P.S. Error began after upgrade from Laravel 8.XX to Laravel 10.