Mail is one of those "forever" problems around here, unfortunately. There are a few things you can do to make sure it works (when it works) from your end, such as making sure that the content is plain text (HTML-formatted MIME mail is not allowed) and that the FROM header reflects an email address that actually exists and is associated with your hosting account (that is, one that you created here). Both of those restrictions are in place to reduce spam and phishing attempts; the server will silently discard HTML mails or mails with spoofed FROM headers. There can occasionally be a volume problem (it happens when you're sharing a server with a few thousand of your closest friends), occasionally the outbound queue will get stuck and needs a kick to release held mail, and every once in a while somebody does something incredibly stupid that taints the x10Hosting SMTP server's reputation, which can wind up with any mail showing signs of being routed through x10Hosting being sent straight to the spam folder, trash, or even silently discarded at the receiving end. And to be clear, this isn't a problem that's unique to x10Hosting or even to free hosting in general.
I don't know if your plugin can be configured to work with an external mail service, but if it can you might want to look at
Mandrill Transactional Email. It's a freemium service, and the chances are pretty darned good that you'd fit comfortably into the free end of freemium with plenty of room to spare (12,000 emails a month ought to do for account activation and forgotten passwords, right?). And while we're on the subject, if a newsletter is in the plans, you could do a lot worse than MailChimp (which is, again, free for small sites/communities -- < 2000 subscribers and 12,000 emails/mo). I'm a bigotted and opinionated hater of WordPress, so I've never investigated the available plugins/APIs, but WP is popular enough that there's likely some kind of off-the-shelf integration available.