A few weeks ago some emails were taking 2-3 days to reach their destination and the problem was an issue between the hosting company and Xtra.
This has happened again today and the email log reads:
May 20 13:04:18 mlx1 postfix-outbound/smtp[20904]: 522AE1AB0030: to=
The hosting company are investigating.
To overcome the problem we set up a POP3 mailbox so that Xtra was bypassed.