minor #10437 Added a caution note about the checkMX option (javiereguiluz)

This PR was merged into the 2.8 branch.

Discussion
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Added a caution note about the checkMX option

Some users on Slack asked for an explicit mention about how unreliable is this option.

Commits
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7910bf4 Added a caution note about the checkMX option

Author: javiereguiluz

reference/constraints/Email.rst

@@ -117,6 +117,11 @@ checkMX
 If true, then the :phpfunction:`checkdnsrr` PHP function will be used to
 check the validity of the MX record of the host of the given email.
 
+.. caution::
+
+    This option is not reliable because it depends on the network conditions
+    and some valid servers refuse to respond to those requests.
+
 checkHost
 ~~~~~~~~~