Most likely, this started after Thunderbird updated from 52.2.1 to 52.3.0, correct? Check the update log in Options > Advanced panel > Update tab > Show Update History if unsure. Does this happen to affect Gmail only? - You could revert to [https://ftp.mozilla.org/pub/thunderbird/releases/52.2.1/win32/en-US/ 52.2.1] to see if that works as before for a quick workaround. That is not a solution, but should proof the issue is update related and still work. You should be able to install that version "on top" of 52.3.0, or (better) uninstall 52.3.0 first. Email and settings should be retained. (See [*] below for creating a backup of your profile first, just in case and which is always good to have.) - You could also try this ''in 52.3.0'': Open the Tools menu (press Alt if needed) > Account settings > <account name> > Server settings -> Advanced… button, and set '''Maximum number of server connections to cache''' to less than 5, such as 4 or 2. (This is only valid for IMAP accounts.) - If this does not help, try clearing Thunderbird’s cache: # Open Help > Troubleshooting Information. # Click '''about:profiles'''. # Look for the profile that says "This is the profile in use and it cannot be deleted." # Below that, next to '''Local Directory''' (NOT ''Root'' directory), click '''Open Folder'''. # Close Thunderbird. # Delete or move the content of the ''Cache'' and ''cache2'' folders, or even all content. # Start Thunderbird and check. [*] Actually, step 3/4 offer a good time to create a backup of your Profile folder. That folder is the one that opens when clicking "Open Folder" next to "Root Directory, i.e. the path that contains "Roaming"". Related/similar questions: [/questions/1173080 here] and [/questions/1173088 here].