I've been using Thunderbird for about a year to manage multiple office365 accounts. Everything worked fine until Saturday when 2 accounts (which are linked to the same place of work) both stopped sending emails. I get the attached error messages. I have been through all the steps on the "cannot-send-messages" thread and checked all settings, reset passwords, deleted passwords from the list of saved passwords. I'm pretty certain I have tried all the solutions already offered and nothing is working. I'm almost certain this is a thunderbird issue as I can send emails if I log into office365. The other 4 office365 mailbox's are working fine though. As its only recently happened, and no settings had been changed prior to the error occurring, I'm wondering if there is something new I need to try? Many thanks in advance for helping me to figure this out!