@buzztone Thank you for posting on these threads about the error message after upgrading to 4.4.1. For us, The new rules released for mod security are not the cause of the issue. We have confirmed with our host running Nginx that they are not using Mod_security.
We’ve deactivated all other plugins including an STMP sending plugin and we are still getting the “Failed to send…” message with WordPress 4.4.1. WordPress 4.4.0 works.
Some users in that previous post mentioned issue related to the the Mail (2) option. We are not using the Mail (2) option.
We had our website name in the From field. We liked having our website name in the From field so that our sales team can more easily see that the email is coming from the website.
Using the default [your-name]<[your-email]> in the From field works but we are hoping that at some point CF7 releases an update that allows us to have our website name in the From field.