Sure, there is no reason to create local system accounts for mail-only users. How you do this will vary some by MTA (mail server).
How many users do you need to provide mail services for? Is this a personal project or something you are doing on a grander scale?
Provide a few more details on the scope of what you hope to achieve and no doubt many will chime in with suggestions.
For both personal mail and fee for service mail I use Postfix as my MTA, Dovecot for "virtual" local delivery / IMAP/POP3, and other bits and pieces. If you are just providing mail services for a handful of users, I'd recommend implementing Postfix/Dovecot using simple text files as your virtual user database(s) rather than opting for a MySQL or PostgreSQL database backend, at least to start with. Doing so removes another chunk of software out of the picture (a SQL db) and may force you to think about the implementation a little more than following one of many recipes that exist out there on the Internet. For my personal / pro bono servers I go this route and use make to rebuild Postfix "database" files (and reload the system when required) when I've made a meaningful edit to the configuration. These systems just keep working year in year out. To make this clear - providing "virtual" mail user support means no local login is required.