[...]
# There is a prefix that will cause messages to be delivered to
folders named verbatim. By default, the prefix is just "+". For
example, to deliver to folder foo.bar, use "+foo.bar@your.server". The
prefix can be changed with the setting "postuser"; for example, if you
have "postuser: me" in your imapd.conf, the address becomes
"me+foo.bar@your.server". (I've also seen "partition+folder.name"
with earlier releases of Cyrus, but haven't checked their
source...) # "account+folder.name@your.server" will get delivered
to folder user.account.folder.name. # plus-less local parts will get
delivered to the INBOX of the account with the same name.
[...]