Changed it to:
# cat /etc/httpd/conf.d/postfixadmin.conf alias /mailadmin /usr/share/postfixadmin <Directory "/usr/share/postfixadmin"> AllowOverride AuthConfig # allow from all Require all granted
</Directory>
Yes, all directories need to have 'Require all granted' on them somewhere - if you look in /etc/httpd/conf/httpd.conf there is the definition of the default document root and in that it has the Require line - that is inherited by all subdirectories so you don't need to explicitly state it. But if you use a directory outside document root, then you need to put it into the definition.
And again if you don't intend to use .htaccess files, then remove the AllowOverride line.
P.