Stephen Wolfram, creator of Mathematica and Wolfram Alpha, has tracked life metrics since 1989, including daily outbound emails. The data are noisy, but averaged by month, works out to about 50 per day, with a mode (most frequent value) of 20-25. /Inbound/ email ranges about 300.
ReCode's Walt Mossberg reports about 350 emails/day.
The NYTimes comment moderation team handle 800 comments/day, each.