You ask the sender why they do that. Seriously.

I assure you that the repliers are not mucking about with the format. They don't have time for such foolishness--and neither do I.

This kind of snark is another reason to move to Outlook (gah).

It is not a snark it is as I see it a valid question. It leaves you fine, they press reply and it comes back mangled..... the change is happening at the other end. So why is it happening? My mail sent from Thunderbird comes back fine. Logically I would be looking for something different being done at the other end. I suggest you ask the repliers and you come back all huffy.

When you post a letter by snail mail and a recipient sends it back shredded you do not start the process of looking for what happened by looking around your desk, you go to where the problem occurred.

The causes could be many, information pasted from Microsoft office products can have weird results and would be one of my first guesses as to a cause. Information passing through some anti virus programs can end up scrambled. Corrupt fonts can see thing go south.

So I repeat ask the people at the other end what they are doing and perhaps with what.

What your seeing is not something I have ever seen described here.... so you looking for something fairly unique to your environment.

When you email yourself and reply to your self do you see it? Is it only people from a certain company? or domain. Perhaps they use an anti spam tool that messes up. Is it only those that reply from outlook and a specific version at that. Is it only those that use Incredimail.