As background regarding the concept of subaddresses, the native and ims-ms channels interpret a + character in the local portion of an address (the mailbox portion) specially: in an address of the form name+subaddress@domain the MTA considers the portion of the mailbox after the plus character a subaddress. The native channel treats a subaddress as additional cosmetic information and actually deliver to the account name, without regard to the subaddress; the ims-ms channel interprets the subaddress as the folder name to which to deliver.
Subaddresses also affect the lookup of aliases by the local channel (that is, the L channel on UNIX) and the lookup of aliases by any channel marked with the aliaslocal keyword, and the lookup of mailboxes by the directory channel. The exact handling of subaddresses for such matching is configurable: when comparing an address against an entry, the MTA always first checks the entire mailbox including the subaddress for an exact match; whether or not the MTA performs additional checks after that is configurable.
The subaddressexact keyword instructs the MTA to perform no special subaddress handling during entry matching; the entire mailbox, including the subaddress, must match an entry in order for the alias to be considered to match. No additional comparisons (in particular, no wildcard comparisons or comparisons with the subaddress removed) are performed. The subaddresswild keyword instructs the MTA that after looking for an exact match including the entire subaddress, the MTA should next look for an entry of the form name+*. The subaddressrelaxed keyword instructs the MTA that after looking for an exact match and then a match of the form name+*, that the MTA should make one additional check for a match on just the name portion. With subaddressrelaxed, an alias entry of the following form matches either name or name+subaddress, transforming a plain name to newname, and transforming name+subaddress to newname+subaddress. The subaddressrelaxed keyword is the default.
Thus the subaddresswild keyword or the subaddressrelaxed keyword may be useful when aliases or a directory channel are in use yet users wish to receive mail addressed using arbitrary subaddresses. These keywords obviate the need for a separate entry for every single subaddress variant on an address.
Note that these keywords only make sense for the local channel (that is, the L channel on UNIX) and the directory channel, or any channel marked with the aliaslocal keyword.
Standard Messaging Server configurations relay upon the L channel indeed having subaddressrelaxed behavior (the default, when other keywords have not been explicitly used).