re : I turned off all devices. This too many cached connections is a message from the server not Thunderbird. It means there are too many opened connections accessing that email address on server. You obviously have Thunderbird accessing the email address/mail account. But perhaps you also have other devices accessing the same account eg: other desktop computers or laptops or phones etc or even another email client on same computer but running in the background and it is accessing the server perhaps without your knowledge. In Thunderbird, you correctly modified the cached connection figure from 5 to 1 for that account. Then you would have need to do the following. Exit Thunderbird. Check to see if you have another email client eg: Outlook which is running in the background on computer. I have come across people who previously used Outlook and did not realise it was auto starting in the background and was still connecting to server. Check the Task Manager to see if something is running. Perhaps start up Outlook and remove the mail account. Then check all other computers and laptops - one person discovered it was a work computer which he could not access at the time because he was working out of the office. Another person shared the email address with his wife and discovered she was accessing the mail account on her own phone causing too many cached connections. Phones - they are one of the most frequent causes of this problem as they have a habit of not closing cached connections. So you would need to switch off the phone for a reasonable length of time in order for the server to realise the connections had been closed.