Dear technician, I have configured my email account to Thunderbird mailbox, but the email sending speed is too slow and cannot be sent successfully. When using Thunderbird mailbox mail merge plug-in, it always shows: Your email has been sent, but due to network or file access errors, the copy could not be placed in the outgoing backup folder (sent). How to set up the account to solve this problem, you can put the email into the outbox or draft box of the email account. How to activate Thunderbird mailbox account enterprise version and enterprise version fee and daily email limit?