I can see that currently there is a problem with Voice delivering emails to a few domains - in particular talktalk.net, virginmedia.com, ntlworld.com, and blueyonder.com.
This is because our outgoing email IP address has been put into a blacklist which these ISPs use. I have requested removal from the blacklist, which usually takes a few days.
I'll update on here when I have more news!
Thanks
Joe