Emails that I send from my Thunderbird client are not showing up when I access my account from a different source (the actual website or other another client via smartphone). It's only for a specific account, my iCloud account (which is using IMAP). I don't know if it's an issue with the actual iCloud server or the Thunderbird client, but all my other accounts seem to be syncing just fine. I tried removing and adding the account again, and it didn't work. If there's anything else that you need to know in order to fix my issue, then please don't hesitate to ask.