Hi Klas
I really love such answers that just miss the point.
Did you test what I said ? Whichever side the plugin is if there is a problem, test it before claim there is no problem :)
We all know validation plugins are Js based so client side. If we encounter issues it may mean there is JS conflicts that breaks the whole process. If I write putting a validation plugin prevents from storing and sending the email, it means there is something that breaks the process.