Hey, thanks for your report!
There has not been a lot of activity here for a while. Is this bug still relevant? Have you managed to find a workaround?

Could I get an answer? If I do not hear anything I will assume this issue is resolved or abandoned. Please get back to me <3

Hey,

I didn't hear anything so I'm going to close it. Feel free to comment if this is still relevant, I can always reopen!

There is a real case related to this issue. When an email is sent through the messenger using alternative mail transport and if this mail transport fails then modified email is pushed to the queue for the next try (without X-Transport header). The second try takes default mail transport whether you want it or not.

In this case I believe the method \Symfony\Component\Mailer\Transport\Transports::send should clone a message before modifying and sending it.

@carsonbot this is still relevant

I've created a separate issue for the email case: #44741

Would you mind sending a PR to the doc @Nommyde or @tjveldhuizen?
Or have a look at the code to see what we can do about that?

Do we know situations in which mutability of the message should be qualified as a feature? Or is it always a bad idea? I think that's also relevant for the kind of addition to the docs.
If no valid use cases are available and we change the code, should we add a feature flag or something like that, to prevent a BC break? Or is the fact that it's mutable a bug, which has to be fixed? A possible fix might be to clone the message payload directly after deserialization and using the original deserialized payload when it's dispatched to the queue again.

I don't think there is any case where mutability of the message is legit.

I don't see what we can fix here.
The transport issue has already been fixed.

Let's close this old issue because it looks like the latest consensus is that we can't/shouldn't do more changes about this. Thanks.