They are changing the contents of someones email without asking them first. That is pretty much the definition of malware. Also, is it even legal?

How would you feel if every text message you sent was appended with “This message has been sent over Verizon Wireless/AT&T/Sprint/T-Mobile and has been checked as not spam. Visit [Carrier URL]”?

It’s find if the footer was an opt-in option, but otherwise you’re advertising for the program without your consent (unless you knew these settings were on by default and are okay with it). The footer also looks unprofessional.

Does it do this by default? Because if so, the proper reaction should be “uninstall”, not “turn it off”. That’s a pretty big violation of trust, IMO.