If you are on a shared virtual server then your IP adress is also shared. That IP address may be blacklisted with the free e-mail service providers, like hotmail, microsoft, google, etc. In that case your e-mails are sent out by the UNA system but get immediately blocked by the freemail servers, even before they get processed by any mail server on their systems. Those e-mails are like ghosts.

To solve that problem, one of the affordable options is to get yourself an e-mail service, that will change your IP adress from that blacklisted to their own system IPs, which they take care for being always whitelisted. But that has been discussed before in this forum several times. I use SMTP2GO and I don't have any issues with blacklisted IP anymore.