"Thanks for your last letter," was rejected. When I typed this, I was thinking that 'det siste brevet' might be similar to 'washing his/her face' (Han/hun vasker ansiktet). Is the inclusion of 'ditt/sitt' required when discussing letters, or can it be implied in the same way 'man vasker ansiktet' implies that one is washing his/her own face?
I don't think this is really the case. "Thanks for the last letter" just means "thanks for the last letter you wrote me", it doesn't mean that the won't follow any more after that. If you say "thanks for the last time" it doesn't mean that you won't ever meet that person again.