Using "user+foo@example.com" 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"user+foo@example.com" style address, the system will first check if there is a .forward+foo file before checking for a .forward file.