I have seen, and I like this better, when people start a new subject and say in the body of the message, that they are changing the subject.  Usually, you know what it relates to, because you have just seen people talking about what the new subjectline becomes, in the old subjectline, which has strayed off topic.  For instance: if somebody sent a message whose subject was:  “watching youtube on my phone”, and the topic strayed into people’s different phones that they used, and whether they were iPhones or android, and the features of each, then somebody starts a new subjectline from scratch and calls it “features or cellphones”, and says in the body:  “I changed the subject so we wouldn’t be off topic” or something like that, you can tell where it came from.  Pam.