cannot send emails

i have several emails, ntl ones

i have suddenly started getting an error message that says it cannot send on the ntl server

no settings, password have been changed recently its just started happening on all devices

i have checked on the blacklist and that all looks ok

i recently had an email from virgin to say my account was blocked but i checked and virgin said it wasn't genuine

im not good with these things, to explanations/help in plain english will greatly help please