We are trying to solve connection issues with virginmedia domains - ntlworld.com, blueyonder.co.uk, virginmedia.com, virgin.net
We have 3,5 million transient errors in the last 7 days while trying to connect to the domains mx records. Emails are delivered in the end, but we always have either connection errors or throttling. IPs are not blacklisted.
Like many ISPs VM may well throttle emails from any given source if the volumes exceed operational limits. Such limits are truly variable dependant upon network loads etc and you are therefore unlikely to be told of specific numbers for hourly/daily/weekly limits.
I am a VM customer and have pointed you at the published information on the subject. You can wait here for a formal response from the VM Forum Team in due course.
We follow the email best practices, and I agree that usually ISPs are not sharing exact numbers on their limits, but in most cases approximate values are provided which can help to optimize the mail sending infrastructure.
There are many cases as well when something should be adjusted from the ISP's side, like increasing thresholds for particular IP ranges, so I would be happy, if someone from VM could have a look.