This only applies to non-literal+ appends. Bad response typically occurs when {size} parameter of append command is larger than server can accept. This prevents TB from sending megabytes of data to a server that has already rejected it.
Tag #1273
This is because it was only used in response to a CAPABILITY imap request. However, often CAPABILITY response occurs without a request so TB sees no need to do a request. Fix by moving the usage of use_literal_plus to where LITERAL+ capability is actually used.
Tag #1273