+PH/06 There is a timeout for writing blocks of data, set by, e.g. data_timeout
+ in the smtp transport. When a block could not be written in a single
+ write() function, the timeout was being re-applied to each part-write.
+ This seems wrong - if the receiver was accepting one byte at a time it
+ would take for ever. The timeout is now adjusted when this happens. It
+ doesn't have to be particularly precise.
+