Most mail sent through the 123-reg Mail Servers is forwarded to other email providers such as Gmail and Hotmail. As this is a shared service, there are a few external factors that can impact of the server’s performance.
As this is a shared service, performance may be limited by the actions of other users. For example, rouge users could cause a build-up of backlogged mail, and can even cause the server to become blacklisted.
There is also the possibility that some users may be using the system to spend spam and other malicious email. In this case, other email systems my see this spam and blacklist the server.
This could cause your emails to be delayed or, in the worst case, bounced back.