Normally setting up users in sendmail is redundant - all you're really setting up is email aliases. So out of the box, if your username is 'henry' you'll be receiving mail for firstname.lastname@example.org, if you also want to receive mail addressed to email@example.com, then you'd add an entry to the virtuser table. Also if you need to handle virtual domains, then you'd add entries here.
Note that in the normal course of events, when the SMTP server receives a message addressed to your domain, if there are no re-writing rules difned in the virtuser/generics tables, then it just passes it on to the delivery agent to work out how to get it to the addressee.
So for a single domain, just adding the users in dovecot is all that's required.
The omplication to this is where you require authentication for SMTP - but in most cases (i.e. unless you're an ISP) its simpler to only allow relaying from your internal LAN/VPN.