If the account is a POP account, a POP account is designed to be accessed with a single email client only. Only one email client at a time can communicate with the incoming mail server. When one email client is communicating with the incoming mail server including when downloading new messages, if a 2nd email client attempts to do the same at the same time, the incoming mail server will temporarily lock out the 2nd email client.
No such problem with an IMAP account which is designed to be accessed with more than one email client.
Regarding sending email, if virgin.net does not have an authenticated SMTP server, you will have problems sending with the account.
Sounds like the account you are using is from the early dark ages of email account access with an email client - a POP account and no authenticated SMTP server.
Hmm, maybe it is an email from the early dark ages. Now, I would like to update, so what is the best email to update to? And is there a way of updating where all the emails that get sent to my old address are pushed to my new address, thus not having to contact everyone and say 'My email has changed'?