This article is from the LAN Mail Protocols FAQ, by John Wobus firstname.lastname@example.org with numerous contributions by others.
There are a number of disadvantages associated with the use of these
* since these have long been no more than a small part of the e-mail
market, software using these methods is often incompatible with
other useful and/or well-known software. A couple of examples are:
+ Use of mail-enabled applications on desktop computers (there
is no fundamental reason that mail software using these
protocols can't provide the API used by mail-enabled
applications, but in general this hasn't come about yet)
+ Use of the usual Unix mail readers & the Unix .forward files.
* since the server is holding mail for the person, the person can
use the server for storage. This leaves the potential for all the
disk-space problems inherent in shared disks: people hogging
disk-space or forgetting to clean up, etc.
* sizing the server: a perennial question people ask is "How big a
machine do I need to serve my campus (or department, or
whatever)". Naturally no one can give a straight answer because it
depends upon so many factors.