CRM Email not working

Hello,

I have trubles with internal email in CRM

It was working for a while (I used it to send invoices directly to customer),
for couple weeks It is not available.

I saw the message that email sent and nothing happend.

2) seond question, i tried to send to leads form email, but it shows
Succesfully sent 0 from 0.

What is the problem with emails?