Wordfence is blocking you from sending mails with JS and CSS.
Some requests contain data that may match patterns the firewall Wordfence uses to detect attacks.
Go to your Wordfence extension menu in "All Options".
In this page go to the Basic FireWall Options part and activate the "Learning Mode".
Verify in the Advanced FireWall Options ...
Looks like the email validation inside CiviCRM does not like that pattern. If it is a valid email address that validation would have to be changed. You can do that yourself with a PR (https://docs.civicrm.org/dev/en/latest/core/contributing/) or at least raise it as an issue (https://docs.civicrm.org/dev/en/latest/tools/issue-tracking/#guidelines). Thanks!