both of those accounts are at the same place and use the same servers. I have seen someone else today here complaining about the same error message with Microsoft's mail offering. I am not sure if it is an issue with the mail servers or an anti virus programs. But I doubt the sudden onset is caused by Thunderbird.

Thanks for your reply.

I thought that it may be something to do with the changes that Microsoft are making to their mail servers although neither of the accounts have been migrated to the new system.

The error is intermittent so I had not thought about it being an AV problem but I will take a look at that.