UBB.Threads sends mail through the phpmailer script instead of the native php mail() function; you should have your webhost check the mail queue for your webspace to validate that messages are being sent and are not just being held on the server.
If some messages get through and others do not, this is likely not a mailserver issue, it is instead likely that your site is blacklisted through one of the many many online authorities (as Ruben said) as some destination servers would read some blacklists and others would not.
