The BufferSize parameter to SMTPAppender has been removed with no replacement. When the SMTPAppender is triggered by an error event it also includes the previous 255 logging events, whether they were errors or not (now that the buffer size is fixed).
By using BufferSize = 1, I was able to send a single error email every time an error occurred with just that error. Now I get completely irrelevant logging events, potentially days old with nothing to do with the error being logged and no simple way to change this.
I'm not sure what version this was introduced - I just upgraded from 0.9.17 to 0.9.24.