|I don't know if you are still looking for it, but after some thorough googling I found this:|
You have to use advance edit - this is the missing key!
What you should is:
1) Delete existing signature (I copy pasted mine to notepad before deleting)
2) In Notepad I deleted all the spaces and breaks so it looked like this: Iamdeletingeverythinginbetween
3) Click New then give it a name with the "blank signature" option
4) Once created click the "Advanced Edit" option. This will open M$ Word to make it.
5) Copy paste that string into Word and this time use normal spaces and for breaks use the shift+return for a soft carriage return. So it will look like this: I am now (shift+return) writing a 2 line signature!!
6)Save the signature, DONT CLOSE Word yet!
7) Click on OK on the options tab in Outlook
8) Make sure you you have the signature appearing for your correct server.
9) Click Apply then totally exit outlook.
10) Close Word.
11) Start Outlook and you now have a signature that won't break when people send you "plain text" emails :)