Implement Message Stream for Postmark Mailer

[fabpot]

Q	A
Feature PR	symfony/symfony#42941
PR author(s)	@driesvints
Merged in	5.4

[carsonbot]

Thank you for this issue.
There has not been a lot of activity here for a while. Has this been resolved?

[alamirault]

Mailers transport bridges are listed here: https://symfony.com/doc/5.0/components/mailer.html#transport with DSN examples.

Likes Notifiers (#18119) WDYT to move mailer bridges docs in repo README.md directly ?

For this example we can explain how option message_stream can be used and impacts

(symfony/symfony#42982 is also specific for Amazon Mailer and it will be logic to have doc directly in the readme)

[mdoutreluingne]

I agree with @alamirault, let's take the same logic as the Notifiers (#18119). However, we must target 5.4 with Mailers transport bridges are listed here : https://symfony.com/doc/5.4/mailer.html#using-a-3rd-party-transport

[alamirault]

I created #18185 and symfony/symfony#49991.
Let's iterate and discuss on this !