Hello Stefano,
First of all thanks for the promt answer. So I tried exactly what you said, and it worked. Configured the SMTP with e-mail on the sites hosting, set the sender name, sender email and return path with the sender email address. I added 5 test subscribers (an address using webmail, two yahoo and gmail address), created a blank theme with two lines and used the "save and test" function. All 5 recieved the mail. One more thing, I modified the vimeo-like theme, changed some colours in the style, and added two custom lines in the bottom, can this affect the delivery?
Now I will try to send out a normal newsletter with the actual theme I'm using. And I will let you know the result. Thanks!
LE: Tried something else instead of sending the normal newsletter, I sent my custom theme only to the testers, none recieved it. Beside this, in the subject I use hungarian characters, and longer title. I guess somethings wrong in this area, it can't be sent maybe because it's too large.
LE: I've tried with the vimeo-like original theme, replaced it, same thing. Shorten the subject to one word, same thing, it isn't delivered. With the vimeo-like theme, either it's the original or my edit, it doesn't get sent. But for example, with the theme-3 it works, it's sent.