"SMS" has proven to confuse some younger users in usertesting. The envelope icon is reserved for email. Is there an established or emerging standard that says "send this by text message"?
I think the use of a speech bubble is fairly common, however android seems to have stuck with an envelope (I'm an iOS man so forgive me if this is wrong)
A quick search shows mostly speech bubble related icons, some using text to further clarify.
If I were designing from scratch I'd consider a speech bubble coming from a mobile phone screen - this shows it's 'chatting' (text message) and that it's something done via mobile.
Quick mockup of that:
To distinguish a SMS message from an Email message iconographically I'd say you need to have that labelled in the icon.
The use of an envelope that is conventionally used on mobile devices, Android at least, groups both SMS's and Emails together, and would therefore not be a suitable choice.
There seems to be a trend with using some sort of speech bubble incorporated into it, looking at some of the SMS apps that are available on android we can clearly see: