We are experiencing the same issue with Mautic 4.0.1 with Amazon SES. The weird thing is that some emails are going through.
We are running a campaign with around 300,000 contacts, and I am seeing the log start to fill up and also noticing lots of failed events.
In the system log I am getting messages like this:
[2021-11-18 18:24:51] mautic.ERROR: [MAIL ERROR] Expected response code 250 but got an empty response (send); kfdsfd@wfdsf.com {"exception":"[object] (Swift_TransportException(code: 0): Expected response code 250 but got an empty response at /var/www/mautic/vendor/swiftmailer/swiftmailer/lib/classes/Swift/Transport/AbstractSmtpTransport.php:447)"} []
in the contact card I see this message and that it has been rescheduled to send later this evening:
Not sure how to go about debugging this. So any ideas would be greatly welcome