Hello Stefano,
I'm currently using your Newsletter plugin (vers. 3.3.7) and I'm facing and issue that seems that can't be solved. I've been using the plugin for about 3 months, everything worked fine, sent 3-4 newsletters and they were recieved. But about a month ago something got stuck and I tried reparing it by myself but no luck. So here's what I have done so far. Have to mention, I have 72 subscribers. When someone subscribes, it gets the welcome e-mail message, every time. If I try in the diagnostic panel to test either the WordPress or Newsletter method, I recieve the e-mail, tried even different destination addreses (yahoo / gmail). I tried to configure the SMTP, with a local e-mail address, where the site is hosted. When I test the SMTP, at the top of the page I get a yellow box, and "Success." indeed I recieve the message. But when I send out a newsletter, compose it, everything is fine, I says sending "1/72" .. "2/72" .. and so on, it finishes "sent 72/72" but It's not recieved, I can relate to this because from that 72, 5 mails are mine. And even in Statistic, 72 sent, and 0 opened. Ok.. I tried a different SMTP from another hosting of mine and gmail, same thing. I can't understand why the test work either way, SMTP test aswell, but newsletter are not sent. And one more thing, sometimes mails are returned by "Mail Deamon" with 550 error high probability of spam. I've checked the email address it isn't blacklisted anywhere, even changed the SMTP and enterd another address as mentioned above. I can't understand. You're plugin would be the best solution, so please help me repair this issue. Best regards!