Unhelpful message on ipad appears saying that smtp send has failed (for smtp.ntlworld.com) ...check settings
changed smtp server to smtp.virginmedia.com. as per instructions here: https://www.virginmedia.com/help/virgin-media-mail-how-to-update-email-settings
test messages to other users in ntlworld domain are delivered ok, even using smtp.ntlworld.com
Message re-sent from icloud account OK
Assume something is changing at the ntlworld / virginmedia message transfer/exchange end??? - unreliable again... switch to Icloud instead?