You should find out which RRD module is required for it to run.
You can see some possible candidates here...
and install it (or them) using How to install CPAN modules
Line 360 of /usr/sbin/mailgraph (if it's not a binary file) may yield some further clues.
Where was this mailgraph utility installed from and when was the last time it worked?
I see 2 possibles in yum search on my 5.10 host, so you may wish to either fix the cpan module manually, or re-install mailgraph, if
yum was used to install in the first place.
You can get a clue to its origin using 2 methods:
rpm -qf /usr/sbin/mailgraph
rpm -qa | grep mailgraph
and use that info to re-install using yum.
If no hits are found, it may have been installed manually, so a manual fix would be indicated.
Please let us know.