Hey!

I see that this is your first PR. That is great! Welcome!

Symfony has acontribution guide which I suggest you to read.

In short:

• Always add tests
• Keep backward compatibility (seehttps://symfony.com/bc).
• Bug fixes must be submitted against the lowest maintained branch where they apply (seehttps://symfony.com/releases)
• Features and deprecations must be submitted against the 6.1 branch.

Review the GitHub status checks of your pull request and try to solve the reported issues. If some tests are failing, try to see if they are failing because of this change.

When two Symfony core team members approve this change, it will be merged and you will become an official Symfony contributor!
If this PR is merged in a lower version branch, it will be merged up to all maintained branches within a few days.

I am going to sit back now and wait for the reviews.

Cheers!

Carsonbot

Headers are not case sensitive. Are you sure it makes a difference?

Some mail clients are reply to our from addressno-reply after upgrading tosymfony mailer and when I check my old mail sent withmailgun-php the only thing different that I find is that, but I would have to review in more depth the behavior of mailgum-php and the handling of the reply-to header

Before:

After:

Closing in favor of#45816
Thanks for letting us know about the issue!