Using "firstname.lastname@example.org" style addresses seems to be disabled in Postfix by default. One single parameter called the recipient_delimiter parameter needs to be set to make use of this kind of addressing in Postfix. Just add this to the main.cf file and reload Postfix:
recipient_delimiter = +
Alternatively, some other characters like '-' or '_' can be used here.
Nice to know when using .forward files in email-delivery; When using a"email@example.com" style address, the system will first check if there is a .forward+foo file before checking for a .forward file.