It's probably because it's a MSdos port of the mail program ,,,
u, 31 Oct
1996, Olaf Schroeder wrote:

> Date: Thu, 31 Oct 1996 08:52:52 -0700
> From: Olaf Schroeder <olaf@digirule.com>
> To: netbsd-help@netbsd.org
> Subject: Username length restriction
> 
> Is there a limit to the length of the login name in NetBSD?
> I don't see any mention in the man pages... but I can't seem
> to get anything longer than 8 characters. If I make a user
> 'catherine', I can't login or e-mail to this account. But if
> I make 'catherin', it's fine.
> 
> I though 8 character limits were restricted to DOS! ;-)
> 
> Am I missing something?
> 
>