Symfony Mailer AWS Load Balancer

Question

I have a php application that uses Symfony Mailer 5.4.0 to send emails to customers and suppliers with attachments like invoices and purchase orders. The emails are sent and received successfully. After the send mail task has completed the system should return to the previous screen and display an informational message notifying the user that the email has sent. This works perfectly in a test environment on servers that are accessed directly by their ip addresses. The problem I am having is with an instance that is running in an Amazon VPC behind a load balancer. On that server the emails send successfully however the user is then presented with a blank screen and has to log back into the system to continue working. It appears to time out before displaying the screen, but I do not receive an http notification or error. I am finding lots of posts on how to set up the Symfony Framework to run behind a load balancer, mostly about configuring it to trust proxies but we are not using the framework, only the Mailer. I have a ticket open with AWS on this. Any help / advice will be greatly appreciated.

I have a configuration file that contains logon information for the O365 email accounts we are using. Everything I am finding in various forums include code to set the framework to trust proxies but none of the folders that are referred to exist in the Mailer configuration.