If this is a text file, it is used verbatim. This option takes some time to complete. It does not create an aliases.dat file, but instead tries to check every address specified by the source text file. That's because non-delivery reports will not be sent to the sender for failures in delivering mail to an aliased address. This is by design.
Courier considers aliases to be private mailing lists.
Because non-delivery notices are not sent, bad addresses will not be immediately detected. Note that the -chk option is really effective for addresses which are local, because there is no real way to determine if a remote mail address is valid. The contents will be the combined contents of all the source files, with all addresses converted to canonical format, with duplicates removed and sub-aliases expanded. This concatenates every non-hidden file in the directory. This temporary file MUST reside on the same filesystem as aliases.dat .