Which file are you storing the mailman mailing list aliases in? If you are storing it in any file but the default (/etc/aliases or /etc/postfix/aliases depending on the installation) you will need to add that file to the aliases map entry in the main.cf file. The entry will end up looking something like
alias_maps = hash:/etc/postfix/aliases, hash:/usr/local/mailman/data/aliases assuming you are storing the mailing list entries in /usr/local/mailman/data/aliases. Otherwise your mailing list aliases will need to be kept in the system default alias file. If you are doing virtual domains use the virtual_alias_maps directive instead.
Once you have determined that the correct alias file is defined in you main.cf file then you will need to reload postfix to use the alias maps described above.
The mailing list entries look like
listname: "| /path/to/mailman listcommand listname" for lists defined as part of the default domain and like
listname@domain: "| /path/to/mailman listcommand listname" for virtual domains. The double quotes around the alias command line are important.
The text alias files are not what postfix actually uses. Instead it typically uses a database format of some sort. Most people use the hash database type for local files. It is also possible to use LDAP and SQL databases for alias maps but it doesn't look like you are. To convert the text alias file into something that postfix can use you will need to run the postalias or postmap command. Look at the man pages (
man postalias and
man postmap) for details but normally the command looks like
Good luck and let me know if I answered the wrong question.