I had the same problem. If you are using an “ntlworld.com” account,
go into settings>mail,contacts,calendars>
select the relevant email then account.
under the incoming server the auto settings have changed it to “imap.ntlworld.com” this needs to be changed to imap.virginmedia.com
Click done & it should all be good.
If you’ve more than one ntlworld account you’ll need to do it for each account