To directly answer the question: man -a (whatever) may be what you're looking for - this will sequentially and interactively bring up all man pages that match the text you enter, but it will likely bring up way more than you actually wanted.
But your problem is different. Your error message indicates that you have no manpages for qmail available.
First, ensure that the appropriate manpage packages are installed via yum. For CentOS 6 these are the packages man man-pages. For 7 and 8, use man-pages man-db man.
If those commands installed anything, try man qmail again.
One thing to note about qmail, is that its documentation is spread out since qmail uses multiple different segmented applications to handle mail delivery. man qmail will bring up the top-level documentation, and therein you'll be given the names of the other programs which have their own pages such as qmail-control and qmail-read.