I had the same problem for months. I am guessing you are using IMAP. I have established, with the VM 2nd line support team, that the problem is related solely to ntlworld.com addresses.
I am advised that the problem is because the ntlworld.com IMAP server has insufficient capacity and so ALL emails were being severely throttled at ALL times. As of last week they implemented a change which only applied the throttling at peak periods (which I was told was 4pm to 11pm) and I can confirm that offpeak the ntlworld emails currently arrive as they should. However at peak, they are still severely throttled the same. I have had a 9.2 MB file take 22mins to download and numerous failures or just part downloads.
I am advised the only solution is for the server to be upgraded and I do not believe there is a date currently envisaged for that. So there is no plan or solution insight.
It does mean that ntlworld.com email users are being treated in a fundamentally inferior way to the other addresses and bearing in mind that ntlworld tends to be the longest standing users it seems rather strange that VM have no plans as far as I am aware to rectify the matter.
If any form administrators have any further info when the server is to be sorted it would be good to know.
The issue is now logged within VM as a "known issue" and every @ntlworld.com email user has the same problem so if it is affecting them they should contact the help line and raise the matter.