It's not clear from the Sinback oral history just how early email was available on GEIS, and I haven't come across anything yet that suggests it was available on DTSS. But Sinback's account clearly describes GEIS as a broadly available computer information system and he claims it supported email. There is sufficient context in the oral history to infer that email was definitely a feature by the mid-1970s.
And Sinback casts some aspersions on Tymshare, suggesting that most of their software was in fact based on GE software. That's outside my scope of inquiry for now, but it is an intriguing question. -- Ian