/usr/portage/categories describes a "mail-server" category, where there is only a mail-mta available in /usr/portage. (Postfix is in mail-mta). Reproducible: Always Steps to Reproduce: 1. 2. 3.
not an error. the mail-server category DOES exist, it just doesn't contain anything right now. some servers that aren't mtas (eg bincimap, courier-imap, ...) will go in there from net-mail.