Bigpond Account on Mac Mail - Default ports timed out

Error on Bigpond email account in Mac Mail "Online Status: Connections to the server "imap.telstra.com" on the default ports timed out". Actions taken to try to solve: Webmail is working, have migrated bigpond to imap server from POP server, have deleted and reloaded bigpond email twice, Telstra are suggesting it's a Mac problem, bigpond email is working on all other devices incl iPad Air, iPad Mini 6 and 2 x android phones, server settings for Bigpond account match both Apple Support and Telstra Support. When Bigpond account is reloaded to Mac, mail is OK for approx 12 hours then error message appears and no mail is being received. Gmail accounts on Mac are all working.

Any suggestions to resolve would be most welcome.