We invite new team members to sign up leantesting but they can not sign up because the verification mail or any e mail is never sent. 

Also when we assign tasks to each other, normally we used to receive a notifier e-mail from leantesting. But it stopped one day (february 23-24th) and since then we don't get any kind of e-mail from leantesting.

I think there is an issue on your mail server. It's interesting it's a still going on issue and I guess you haven't even realize it yet.