Symfony version(s) affected

6.0.19

Description

Sending Emails within the same process shortly after each other to the same recipient address randomly produces this error:

Expected response code "354" but got code "250", with message "250 2.1.5 Ok"

Resending the email later produces no error any more.

The operator of the SMTP Server says: "It has to do with pipelining. Your SMTP client is probably mashing up messages from SMTP server with different emails being sent."

Is there any chance that he is right?

How to reproduce

The event is random, approximatively once every 1000 emails - I have not found a possibility to intentionally reproduce it.

Possible Solution

No response

Additional Context

A worker process (continuously running) looks for emails to be sent. Each email is sent serial, synchroneous.