re :''Three days ago "underlivered" mail started coming and now there are over 1,000 and I never sent them in the first place. '' This sounds like someone has abused your email address to send messages. You only discover this when some do not get delivered and you get a bounce back message from server. It is bounced back to your email address as the server 'thinks' it is from you based on the 'From email address. You cannot stop nefarious people from abusing your email address. Usually, they get bored after a while if they do not get responses. They may have included something in the content, possibly a link, trying to get the recipients to respond, so they can detect if the email address is good or other nefarious purposes. It is unlikely they did this by accessing your webmail account but it is possible. If they accessed the account and changed something like the password, then Thunderbird is not going to get access because it is using the original password you setup. re : ''This morning my inbox was blocked I assume by Thunderbird.'' If the server eg: Optus decided you/ your email address was sending out a load of spam based on all the bounced back emails, then Optus could have locked the account. Try to logon to Optus webmail account via a browser. Do you get access to webmail account? Can you see new incoming mail that has not been downloaded to Thunderbird? Are you able to send an email using the webmail account? Please report back on results. In Thunderbird: When you try to 'get messages', what happens ? What does it say in lower Status Bar ...logon, connecting, connected, etc? Do you get an error message? If yes, what does it say word for word? Are you able to send an email fromThunderbird optus account? If no, do you get an error message and if yes, what does it say word for word ?