Hi there,
This is due to the ios devices, I see it quite a lot. The ios device send plain text and exchange won't reformat the mail to HTML and as such will screwup your HTML signature.
The iPhone/iPad running 4 or later defaults to HTML unless you receive a plain text email. I've hunted around and there doesn't seem to be any settings to control this on ios.
Can you supply some more information like what version of ios the devices affected are running and how the accounts are configure (presuming exchange active sync but you could have used Imap)
I know how to do this on premise but I'm looking for a cmdlet to use in power shell that would help.
Ill get back to you.
Graham