I would argue the majority of email users back in the 90's were actually using Unix clients like Pine. Some of the first major users of email in those times were college students, all universities setup email accounts for each student, hundreds of thousands of daily email users, which could be checked at dedicated terminals in libraries, etc. Those were usually Pine. Just think of those many thousands of users all knowing the Pine keyboard shortcuts to send an email -- I think I can still remember them, actually. There was no mouse.