why, I have no idea and neither does anyone else. The error occurred between your computer and I assume Telstra's mail server (with the usual interference by an antivirus program that thinks scanning mail is important I would guess. What anyone here can tell you is what is in the screen shot you provided. I do know there have been a number of posts here recently from Telstra customers. So my guess is they are delivering their usual sterling product that sort of works sometimes. Like my mobile service from them really. Or it could be they have finally retired the Bigpond named mail servers. Have you actually checked your outgoing server (SMTP) in account settings against the setting provided by Telstra. https://www.telstra.com.au/support/email/imap-pop-smtp-mail-server-settings