https://support.mozilla.org/en-US/kb/problems-sending-and-receiving-messages

Thanks christ1 - I checked all the settings and everything seems to be correct, though it still doesn't work.

Was the libero.it email account provided by your ISP? (I guess not; I don't read Italian but it looks like a stand-alone email service.)

Who was your ISP before virgin media?

No. The libero account was independent from my previous isp, which was telecom italia. I believe libero is part of yahoo and operates in italy.

There's no sign of yahoo (from a cursory glance) on the libero website, but that's not to say their email isn't contracted out to or underwritten by yahoo.

Yahoo has been infamous for its limitations on access; it varies from region to region, and for some time they pretended that IMAP wasn't available when it was. Some yahoo accounts were set up for webmail only and you had to pay (Yahoo! Plus!) for officially sanctioned POP access. It might work if set up to use IMAP. ;-)

It's possible that telecom italia gave you an IP address that resolved to italy and so was whitelisted, but the presumably UK-based IP address that virgin will give you may not be allowed.

I can't see any indication of an English language variant of the libero website, so it's possible they don't seek or support business from outside of Italy. :-(

Thanks Zenos - I tried setting up IMAP but it gave me the same error message. I think you're right, in that it's an IP issue.