I guess it's resolved.

In send_update_notification() we send mail in a loop trying up to 5 times if an error occurrs. But this is the only place where we do this. I think it would make sense to do this in all cases, i.e. in _sendmail(). I see also we don't call smtp.quit() consistently. Once we do this on success only, once we do this on error too.

Just search for tickets with "china".

@Laura Goto Settings > Folders. See checkboxes (switches) on the list? These are subscription change switches. Now imagine we add multi-selection capability to that list, which would add a normal checkbox on the left of each folder name. If we do not use the blue-switch-element we end up with two unlabelled checkboxes on each record. So, styling one of them differently would be less confusing. Though still not perfect, that's only one reason. And as I've said I don't have a strong opinion.

We're limited by Font-Awesome iconset. And there the tasks icon is https://fontawesome.com/icons/tasks?style=solid. I also am not sure if only the icon change would fix the issue.

Looks to me like now you'll add user/ prefix twice. See in user_mailbox_extsts() and code above your change.