E-Mail accounts take up disk space and also uses bandwidth resources.
The truth is 'There is no such thing as unlimited e-mail A/C's'
Some Web Hosting firms offer Unlimited provided you have your own in house mail server, where all e-mail is dumped to your own server and then sorted out.
The more POP3 accounts you have, will potentially increase the amount of disk storage. This factor is dependant on how often your clear your e-mail off the server.