According to Sifted by the Financial Times, in brand and product naming the word “email” causes stress responses 
Then there’s the heap of research around the emotional impact of certain word categories; fruit names are apparently calming (Apple, Blackberry), while “email” increases stress responses.
They’re quoting the New Yorker, which quotes a company called Lexicon, which is the brand consultancy which gave Blackberry its name.
The research may have been in the 90s, but I’m guessing people’s reactions today are still similar.
Personally I find WhatsApp and other IM more stressful because it comes with expectations of a more immediate response. Emails feel more under my control. Perhaps that’s just me 