minor #15467 [Mailer] Recommending "strict" mode for users of Symfony Mailer (ThomasLandauer)

This PR was merged into the 4.4 branch.

Discussion
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[Mailer] Recommending "strict" mode for users of Symfony Mailer

Maybe you could add a link to Symfony Mailer?

The information is taken from symfony/symfony#41850 (comment)

Commits
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e23c7ec Recommending "strict" mode for users of Symfony Mailer

Author: javiereguiluz

reference/constraints/Email.rst

@@ -153,8 +153,10 @@ in the second host part of the email address.
 strict
 ......
 
-Uses the `egulias/email-validator`_ library to perform an RFC compliant
-validation. You will need to install that library to use this mode.
+If you're using Symfony Mailer to send emails to that address, this is
+the recommended mode, since both are using the same library, to perform an RFC
+compliant validation: `egulias/email-validator`_. If you're not using Symfony
+Mailer, you will need to install that library separately to use this mode.
 
 html5
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