Why can t-bird retain formatting as outlook express does?
I often send messages to multiple correspondents, get replies and forward or respond to those replies. Perhaps t--bird sends my original message as formatted, but when it comes back with a response, the formatting is gone and trying to clean it up is a mess.
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So the responses to you have been mangled by some other email client, have they not?
Just check the setting in On the toolbar 
> options > Composition > send options
Is set to both plain text and HTML.
If that fails, you might try HTML only, if your recipient can not read HTML (like they still use Eudora) then they will get a message that is a mess of HTML codes. So both is a preferable setting.
It was set for HTML only, which is what our son uses (he's used t-bird for years) and he has the same problem. I changed it to "both" and we'll see what happens.
Zenos said
So the responses to you have been mangled by some other email client, have they not?
Probably not, same problem. t-bird to t-bird. Between my wife's computer and mine, both having the same settings, and my son, who's smart and a long time t-bird user.