Alfresco can be used for this purpose, but it won't be as fast as a dedicated mail server. It does a lot of useful things with the content in addition to storing it, such as attaching metadata, executing rules, running workflows, and search indexing.
In most cases, people would only send a subset of their emails to Alfresco for which they need these additional capabilities. But if it is properly configured for performance, it could handle all the email for a moderately sized organization.
But if you only need to store email, it is probably better to use something designed for that purpose.